diff --git a/python/compare_dqm.py b/python/compare_dqm.py
new file mode 100644
index 0000000..7c1adf1
--- /dev/null
+++ b/python/compare_dqm.py
@@ -0,0 +1,579 @@
+#!/usr/bin/env python
+"""Dash app that builds a dataset dropdown from metacat output.
+
+This is a pared-down version of :mod:`DqmMenu` intended to launch a
+web page on port 8050 and simply display the dataset names in a
+<select> control.  The underlying dataset query is:
+
+    metacat dataset list mu2e:ntd.*
+
+Names are stripped of the leading ``mu2e:ntd.mu2e.`` prefix for
+readability.
+"""
+
+from __future__ import annotations
+
+import base64
+import os
+import subprocess
+import tempfile
+import uuid
+import webbrowser
+from pathlib import Path
+
+try:
+    from dash import Dash, Input, Output, callback, dcc, html
+except ImportError as exc:
+    raise SystemExit("Failed to import dash. Install it in your runtime environment.") from exc
+
+DATASET_CMD = 'metacat dataset list "mu2e:dqm.mu2e.*"'
+PREFIX = "mu2e:dqm.mu2e."
+MISSING_COMPARE_HISTOGRAM_MESSAGE = "histogram does not exist in chosen file"
+
+
+def load_datasets():
+    """Return ``(datasets, status)`` where ``datasets`` is a list of
+    cleaned-up names and ``status`` is a human-readable message.
+    """
+
+    try:
+        result = subprocess.run(
+            ["bash", "-lc", DATASET_CMD],
+            check=True,
+            capture_output=True,
+            text=True,
+        )
+    except FileNotFoundError:
+        return [], "bash is not available in this environment."
+    except subprocess.CalledProcessError as exc:
+        stderr = exc.stderr.strip() or exc.stdout.strip() or "Unknown error"
+        return [], f"Dataset query failed: {stderr}"
+
+    raw = [line.strip() for line in result.stdout.splitlines() if line.strip()]
+    if not raw:
+        return [], "No datasets returned by metacat."
+
+    cleaned = []
+    for ds in raw:
+        if ds.startswith(PREFIX):
+            cleaned.append(ds[len(PREFIX):])
+        else:
+            cleaned.append(ds)
+
+    return cleaned, f"Loaded {len(cleaned)} datasets."
+
+
+def load_files(dataset_full):
+    """Return ``(files, status)`` for the given full dataset name."""
+    cmd = f'metacat dataset files "{dataset_full}" | sort | mdh print-url -l disk -'
+    try:
+        result = subprocess.run(
+            ["bash", "-lc", cmd],
+            check=True,
+            capture_output=True,
+            text=True,
+        )
+    except FileNotFoundError:
+        return [], "bash is not available in this environment."
+    except subprocess.CalledProcessError as exc:
+        stderr = exc.stderr.strip() or exc.stdout.strip() or "Unknown error"
+        return [], f"File query failed: {stderr}"
+
+    files = [line.strip() for line in result.stdout.splitlines() if line.strip()]
+    if not files:
+        return [], "No files returned."
+
+    return files, f"Loaded {len(files)} files."
+
+
+def render_histogram_image(
+    root_path: str | Path,
+    hist_name: str,
+    compare_path: str | Path | None = None,
+    ks_test: bool = False,
+) -> str:
+    try:
+        import ROOT
+    except Exception as exc:  # pragma: no cover - runtime dependency check
+        raise RuntimeError(
+            "Missing dependency: PyROOT is required. "
+            "Ensure ROOT is installed and available in your Python environment."
+        ) from exc
+
+    def normalize(path: str | Path) -> str:
+        s = str(path)
+        if s.startswith("file://"):
+            return s[len("file://") :]
+        return s
+
+    primary_str = normalize(root_path)
+    compare_str = normalize(compare_path) if compare_path is not None else None
+
+    # Root can open xrootd/http URLs directly. Only validate local paths.
+    if not (primary_str.startswith("root://") or primary_str.startswith("http://") or primary_str.startswith("https://")):
+        if not Path(primary_str).exists():
+            raise FileNotFoundError(f"ROOT file not found: {primary_str}")
+
+    ROOT.gROOT.SetBatch(True)
+    try:
+        primary_file = ROOT.TFile.Open(primary_str, "READ")
+    except TypeError:
+        primary_file = ROOT.TFile.Open(primary_str)
+
+    if not primary_file or primary_file.IsZombie():
+        raise RuntimeError(f"Unable to open ROOT file: {primary_str}")
+
+    compare_file = None
+    try:
+        clone_suffix = uuid.uuid4().hex
+
+        def clone_histogram(source_file, source_name, clone_name, missing_message):
+            source_obj = source_file.Get(source_name)
+            if not source_obj:
+                raise RuntimeError(missing_message)
+            if not (source_obj.InheritsFrom("TH1") or source_obj.InheritsFrom("TProfile")):
+                raise RuntimeError(f"Object is not a histogram: {source_name}")
+
+            cloned_obj = source_obj.Clone(clone_name)
+            if hasattr(cloned_obj, "SetDirectory"):
+                cloned_obj.SetDirectory(0)
+            return cloned_obj
+
+        primary_obj = clone_histogram(
+            primary_file,
+            hist_name,
+            f"primary_histogram_to_draw_{clone_suffix}",
+            f"Histogram not found: {hist_name}",
+        )
+
+        compare_obj = None
+        if compare_str:
+            # Only validate local compare file paths, allow xrootd/http to pass through.
+            if not (compare_str.startswith("root://") or compare_str.startswith("http://") or compare_str.startswith("https://")):
+                if not Path(compare_str).exists():
+                    raise FileNotFoundError(f"Compare ROOT file not found: {compare_str}")
+
+            try:
+                compare_file = ROOT.TFile.Open(compare_str, "READ")
+            except TypeError:
+                compare_file = ROOT.TFile.Open(compare_str)
+
+            if not compare_file or compare_file.IsZombie():
+                raise RuntimeError(f"Unable to open compare ROOT file: {compare_str}")
+
+            compare_obj = clone_histogram(
+                compare_file,
+                hist_name,
+                f"compare_histogram_to_draw_{clone_suffix}",
+                MISSING_COMPARE_HISTOGRAM_MESSAGE,
+            )
+
+        canvas = ROOT.TCanvas("hist_canvas", "Histogram", 900, 650)
+        primary_obj.SetLineColor(ROOT.kBlack)
+        primary_obj.SetMarkerColor(ROOT.kBlack)
+        primary_obj.SetLineWidth(2)
+
+        if compare_obj is not None:
+            max_y = max(primary_obj.GetMaximum(), compare_obj.GetMaximum())
+            if max_y > 0:
+                primary_obj.SetMaximum(max_y * 1.15)
+
+        primary_obj.Draw("HIST")
+
+        if compare_obj is not None:
+            compare_obj.SetLineColor(ROOT.kBlue)
+            compare_obj.SetMarkerColor(ROOT.kBlue)
+            compare_obj.SetLineWidth(2)
+            compare_obj.Draw("HIST SAME")
+
+            legend = ROOT.TLegend(0.37, 0.78, 0.63, 0.9)
+            legend.SetBorderSize(0)
+            legend.SetFillStyle(0)
+            legend.SetTextSize(0.02)
+            legend.AddEntry(primary_obj, "Selected file", "l")
+            legend.AddEntry(compare_obj, "Compare file", "l")
+            legend.Draw()
+
+            if ks_test:
+                ks_pvalue = primary_obj.KolmogorovTest(compare_obj)
+                ks_text = ROOT.TLatex()
+                ks_text.SetNDC(True)
+                ks_text.SetTextSize(0.035)
+                # Show at least 4 digits of precision for the p-value.
+                ks_text.DrawLatex(0.12, 0.92, f"KS-test p-value: {ks_pvalue:.4g}")
+
+        with tempfile.NamedTemporaryFile(suffix=".png", delete=False) as tmp:
+            tmp_path = tmp.name
+        try:
+            canvas.SaveAs(tmp_path)
+            with open(tmp_path, "rb") as f:
+                encoded = base64.b64encode(f.read()).decode("ascii")
+        finally:
+            canvas.Close()
+            if os.path.exists(tmp_path):
+                os.remove(tmp_path)
+        return f"data:image/png;base64,{encoded}"
+    finally:
+        primary_file.Close()
+        if compare_file:
+            compare_file.Close()
+
+
+def load_histogram_names(root_path: str | Path) -> list[str]:
+    try:
+        import ROOT
+    except Exception as exc:  # pragma: no cover - runtime dependency check
+        raise RuntimeError(
+            "Missing dependency: PyROOT is required. "
+            "Ensure ROOT is installed and available in your Python environment."
+        ) from exc
+
+    path_str = str(root_path)
+
+    # Allow ROOT to handle xrootd and other non-filesystem paths directly.
+    if path_str.startswith("file://"):
+        path_str = path_str[len("file://") :]
+
+    if not (path_str.startswith("root://") or path_str.startswith("http://") or path_str.startswith("https://")):
+        if not Path(path_str).exists():
+            raise FileNotFoundError(f"ROOT file not found: {path_str}")
+
+    histogram_names: list[str] = []
+    try:
+        root_file = ROOT.TFile.Open(path_str, "READ")
+    except TypeError:
+        # Some ROOT/PyROOT bindings only accept a single string argument.
+        root_file = ROOT.TFile.Open(path_str)
+
+    if not root_file or root_file.IsZombie():
+        raise RuntimeError(f"Unable to open ROOT file: {path_str}")
+
+    def walk_dir(directory: "ROOT.TDirectory", prefix: str = "") -> None:
+        for key in directory.GetListOfKeys():
+            name = key.GetName()
+            obj = key.ReadObj()
+            if obj.InheritsFrom("TDirectory"):
+                walk_dir(obj, f"{prefix}{name}/")
+                continue
+            if obj.InheritsFrom("TH1") or obj.InheritsFrom("TProfile"):
+                histogram_names.append(f"{prefix}{name}")
+
+    try:
+        walk_dir(root_file)
+    finally:
+        root_file.Close()
+
+    histogram_names.sort()
+    return histogram_names
+
+
+def build_app():
+    app = Dash(__name__)
+
+    initial_datasets, initial_status = load_datasets()
+    initial_value = initial_datasets[0] if initial_datasets else None
+
+    app.layout = html.Div(
+        [
+            html.Img(
+                id="histogram-image",
+                style={
+                    "maxWidth": "100%",
+                    "border": "1px solid #ccc",
+                    "padding": "0.5rem",
+                    "background": "#fff",
+                    "marginBottom": "20px",
+                },
+            ),
+            html.H2("Mu2e Datasets"),
+            html.P(
+                "Entries come from the local shell command ``{}``.".format(DATASET_CMD)
+            ),
+            html.Div(
+                [
+                    html.Button("Refresh", id="refresh-button", n_clicks=0),
+                    html.Div(
+                        initial_status,
+                        id="status-message",
+                        style={"marginTop": "12px", "fontFamily": "monospace"},
+                    ),
+                ],
+                style={"marginBottom": "18px"},
+            ),
+            dcc.Dropdown(
+                id="dataset-dropdown",
+                options=[{"label": name, "value": name} for name in initial_datasets],
+                value=initial_value,
+                placeholder="Select a dataset",
+                style={"maxWidth": "900px"},
+            ),
+            html.Pre(
+                initial_value or "No dataset selected.",
+                id="selected-dataset",
+                style={
+                    "marginTop": "20px",
+                    "padding": "12px",
+                    "backgroundColor": "#f2f2f2",
+                    "border": "1px solid #cccccc",
+                    "maxWidth": "900px",
+                    "whiteSpace": "pre-wrap",
+                },
+            ),
+            html.H3("Files in Selected Dataset"),
+            dcc.Dropdown(
+                id="file-dropdown",
+                options=[],
+                value=None,
+                placeholder="Select a file",
+                style={"maxWidth": "900px"},
+            ),
+            html.Pre(
+                "No file selected.",
+                id="selected-file",
+                style={
+                    "marginTop": "20px",
+                    "padding": "12px",
+                    "backgroundColor": "#f2f2f2",
+                    "border": "1px solid #cccccc",
+                    "maxWidth": "900px",
+                    "whiteSpace": "pre-wrap",
+                },
+            ),
+            html.H3("Histograms in Selected File"),
+            dcc.Dropdown(
+                id="histogram-dropdown",
+                options=[],
+                value=None,
+                placeholder="Select a histogram",
+                style={"maxWidth": "900px"},
+            ),
+            html.Div(
+                "",
+                id="histogram-status",
+                style={"marginTop": "12px", "fontFamily": "monospace"},
+            ),
+            html.Pre(
+                "No histogram selected.",
+                id="selected-histogram",
+                style={
+                    "marginTop": "20px",
+                    "padding": "12px",
+                    "backgroundColor": "#f2f2f2",
+                    "border": "1px solid #cccccc",
+                    "maxWidth": "900px",
+                    "whiteSpace": "pre-wrap",
+                },
+            ),
+            html.H3("Compare Dataset"),
+            dcc.Dropdown(
+                id="compare-dataset-dropdown",
+                options=[{"label": name, "value": name} for name in initial_datasets],
+                value=None,
+                placeholder="Select a compare dataset",
+                style={"maxWidth": "900px"},
+            ),
+            html.Pre(
+                "No compare dataset selected.",
+                id="selected-compare-dataset",
+                style={
+                    "marginTop": "20px",
+                    "padding": "12px",
+                    "backgroundColor": "#f2f2f2",
+                    "border": "1px solid #cccccc",
+                    "maxWidth": "900px",
+                    "whiteSpace": "pre-wrap",
+                },
+            ),
+            html.H3("Compare File"),
+            dcc.Dropdown(
+                id="compare-file-dropdown",
+                options=[],
+                value=None,
+                placeholder="Select a compare file",
+                style={"maxWidth": "900px"},
+            ),
+            html.Div(
+                [
+                    dcc.Checklist(
+                        id="compare-enabled",
+                        options=[{"label": "Compare", "value": "compare"}],
+                        value=[],
+                    ),
+                    dcc.Checklist(
+                        id="ks-test-enabled",
+                        options=[{"label": "KS-test", "value": "ks"}],
+                        value=[],
+                    ),
+                ],
+                style={
+                    "display": "flex",
+                    "alignItems": "center",
+                    "gap": "1rem",
+                    "marginTop": "8px",
+                    "marginBottom": "20px",
+                },
+            ),
+            html.Div(
+                "",
+                id="comparison-status",
+                style={
+                    "marginTop": "12px",
+                    "fontFamily": "monospace",
+                    "color": "#b00020",
+                },
+            ),
+            html.Pre(
+                "No compare file selected.",
+                id="selected-compare-file",
+                style={
+                    "marginTop": "20px",
+                    "padding": "12px",
+                    "backgroundColor": "#f2f2f2",
+                    "border": "1px solid #cccccc",
+                    "maxWidth": "900px",
+                    "whiteSpace": "pre-wrap",
+                },
+            ),
+        ],
+        style={"padding": "20px", "fontFamily": "Helvetica, Arial, sans-serif"},
+    )
+
+    @callback(
+        Output("dataset-dropdown", "options"),
+        Output("dataset-dropdown", "value"),
+        Output("compare-dataset-dropdown", "options"),
+        Output("compare-dataset-dropdown", "value"),
+        Output("status-message", "children"),
+        Input("refresh-button", "n_clicks"),
+    )
+    def refresh(_n_clicks):
+        datasets, status = load_datasets()
+        options = [{"label": name, "value": name} for name in datasets]
+        value = datasets[0] if datasets else None
+        return options, value, options, None, status
+
+    @callback(Output("selected-dataset", "children"), Input("dataset-dropdown", "value"))
+    def show_selection(dataset_name):
+        return dataset_name or "No dataset selected."
+
+    @callback(
+        Output("selected-compare-dataset", "children"),
+        Input("compare-dataset-dropdown", "value"),
+    )
+    def show_compare_dataset_selection(dataset_name):
+        return dataset_name or "No compare dataset selected."
+
+    @callback(
+        Output("file-dropdown", "options"),
+        Output("file-dropdown", "value"),
+        Input("dataset-dropdown", "value"),
+    )
+    def update_files(dataset_name):
+        if not dataset_name:
+            return [], None
+        dataset_full = PREFIX + dataset_name
+        files, _ = load_files(dataset_full)
+        options = [{"label": os.path.basename(url), "value": url} for url in files]
+        value = files[0] if files else None
+        return options, value
+
+    @callback(
+        Output("compare-file-dropdown", "options"),
+        Output("compare-file-dropdown", "value"),
+        Input("compare-dataset-dropdown", "value"),
+    )
+    def update_compare_files(dataset_name):
+        if not dataset_name:
+            return [], None
+        dataset_full = PREFIX + dataset_name
+        files, _ = load_files(dataset_full)
+        options = [{"label": os.path.basename(url), "value": url} for url in files]
+        value = files[0] if files else None
+        return options, value
+
+    @callback(Output("selected-file", "children"), Input("file-dropdown", "value"))
+    def show_file_selection(file_url):
+        return file_url or "No file selected."
+
+    @callback(Output("selected-compare-file", "children"), Input("compare-file-dropdown", "value"))
+    def show_compare_file_selection(file_url):
+        return file_url or "No compare file selected."
+
+    @callback(
+        Output("histogram-dropdown", "options"),
+        Output("histogram-dropdown", "value"),
+        Output("histogram-status", "children"),
+        Input("file-dropdown", "value"),
+    )
+    def update_histograms(file_url):
+        if not file_url:
+            return [], None, ""
+
+        # ROOT can open xrootd paths directly, but if we get a file:// URL we
+        # need to strip the scheme for local disk paths.
+        if file_url.startswith("file://"):
+            file_url = file_url[len("file://") :]
+
+        try:
+            names = load_histogram_names(file_url)
+        except Exception as exc:
+            return [], None, f"Error loading histograms: {exc}"
+
+        if not names:
+            return [], None, "No histograms found in file."
+
+        options = [{"label": name, "value": name} for name in names]
+        value = names[0]
+        return options, value, f"Loaded {len(names)} histograms."
+
+    @callback(Output("selected-histogram", "children"), Input("histogram-dropdown", "value"))
+    def show_histogram_selection(hist_name):
+        return hist_name or "No histogram selected."
+
+    @callback(
+        Output("histogram-image", "src"),
+        Output("comparison-status", "children"),
+        Input("file-dropdown", "value"),
+        Input("compare-file-dropdown", "value"),
+        Input("compare-enabled", "value"),
+        Input("ks-test-enabled", "value"),
+        Input("histogram-dropdown", "value"),
+    )
+    def update_histogram_image(file_url, compare_file_url, compare_enabled, ks_test_enabled, hist_name):
+        if not file_url or not hist_name:
+            return "", ""
+
+        compare_path = None
+        ks_test = False
+        if compare_file_url and compare_enabled and "compare" in compare_enabled:
+            compare_path = compare_file_url
+            ks_test = bool(ks_test_enabled and "ks" in ks_test_enabled)
+
+        try:
+            return render_histogram_image(file_url, hist_name, compare_path, ks_test), ""
+        except RuntimeError as exc:
+            if str(exc) == MISSING_COMPARE_HISTOGRAM_MESSAGE:
+                print(MISSING_COMPARE_HISTOGRAM_MESSAGE)
+                return (
+                    render_histogram_image(file_url, hist_name),
+                    MISSING_COMPARE_HISTOGRAM_MESSAGE,
+                )
+            raise
+
+    return app
+
+
+def main():
+    app = build_app()
+    # ROOT / PyROOT is not thread-safe, so run the server single-threaded.
+    app.run(
+        host="127.0.0.1",
+        port=8050,
+        debug=False,
+        threaded=False,
+        use_reloader=False,
+    )
+
+
+if __name__ == "__main__":
+    main()