diff --git a/docs/source/tendencies.rst b/docs/source/tendencies.rst
index 88030e0e..88458147 100644
--- a/docs/source/tendencies.rst
+++ b/docs/source/tendencies.rst
@@ -51,6 +51,28 @@ If the ``value`` is not specified, it will be set to the last value of the previ
     - {type: linear, to: 3, duration: 10}
     - {type: constant, duration: 10}
 
+Value types
+-----------
+
+The ``value`` is type-aware and is not restricted to floating-point numbers:
+
+*   **Numeric values** (floats and integers) produce a floating-point signal that is
+    interpolated across gaps between tendencies, as usual. Integers are treated as
+    floats during evaluation.
+*   **Categorical values** (strings or booleans) describe non-numeric signals, such
+    as a heating scheme (``"nbi"``, ``"ec"``) or an on/off flag. These are held as a
+    step (zero-order hold) rather than interpolated: across a gap, the previous value
+    is carried forward, and the same applies when extrapolating beyond the waveform.
+
+.. code-block:: yaml
+
+    - {type: constant, value: ohmic, duration: 2}
+    - {type: constant, value: nbi, duration: 2}
+
+.. note::
+    Categorical and numeric values cannot be mixed within a single waveform, because
+    the gaps between them cannot be interpolated. Doing so raises a validation error.
+
 Linear Tendency
 ===============
 
diff --git a/tests/tendencies/test_constant.py b/tests/tendencies/test_constant.py
index c21f87cf..b557dfec 100644
--- a/tests/tendencies/test_constant.py
+++ b/tests/tendencies/test_constant.py
@@ -86,3 +86,49 @@ def test_declarative_assignments():
     assert t2.value == 6
     assert not t1.annotations
     assert not t2.annotations
+
+
+def test_integer_value_not_coerced():
+    """Integer inputs are kept as integers, not coerced to float."""
+    tendency = ConstantTendency(user_duration=1, user_value=5)
+    assert tendency.value == 5
+    assert isinstance(tendency.value, int)
+    assert not tendency.is_categorical
+    assert not tendency.annotations
+
+
+def test_string_value():
+    """A constant tendency can hold a string (non-numeric) value."""
+    tendency = ConstantTendency(user_duration=2, user_value="nbi")
+    assert tendency.value == "nbi"
+    assert tendency.is_categorical
+    assert tendency.start_value == "nbi"
+    assert tendency.end_value == "nbi"
+
+    _, values = tendency.get_value(np.array([0.0, 1.0, 2.0]))
+    assert list(values) == ["nbi", "nbi", "nbi"]
+    assert not tendency.annotations
+
+
+def test_boolean_value():
+    """A constant tendency can hold a boolean value, kept as a (categorical) bool."""
+    tendency = ConstantTendency(user_duration=2, user_value=True)
+    assert tendency.value is True
+    assert tendency.is_categorical
+    # start/end values round-trip through numpy, so compare by value (np.bool_)
+    assert bool(tendency.start_value) is True
+    assert bool(tendency.end_value) is True
+
+    _, values = tendency.get_value(np.array([0.0, 1.0, 2.0]))
+    assert [bool(v) for v in values] == [True, True, True]
+    assert values.dtype == bool
+    assert not tendency.annotations
+
+
+def test_string_value_chained():
+    """A value-less string constant inherits the previous string value."""
+    prev = ConstantTendency(user_value="ec", user_start=0, user_duration=1)
+    tendency = ConstantTendency(user_duration=1)
+    tendency.set_previous_tendency(prev)
+    assert tendency.value == "ec"
+    assert tendency.is_categorical
diff --git a/tests/test_waveform.py b/tests/test_waveform.py
index 6735d8a3..2786e760 100644
--- a/tests/test_waveform.py
+++ b/tests/test_waveform.py
@@ -273,3 +273,69 @@ def test_overlap_derivatives():
     expected = [2, 2, -1.5, -1.5, -1.5, -1.5, -1.5]
     values = waveform.get_derivative(np.linspace(0, 3, 7))
     assert np.allclose(values, expected)
+
+
+def test_string_waveform():
+    """A waveform of string constants evaluates as a zero-order-hold step function."""
+    waveform = Waveform(
+        waveform=[
+            {"user_type": "constant", "user_value": "ohmic", "user_duration": 2},
+            {"user_type": "constant", "user_value": "nbi", "user_duration": 2},
+        ]
+    )
+    assert waveform.is_categorical
+
+    _, values = waveform.get_value(np.array([0.0, 1.0, 2.0, 3.0]))
+    # The switch happens at t=2; later tendencies take precedence at the boundary.
+    assert list(values) == ["ohmic", "ohmic", "nbi", "nbi"]
+
+    # Values are held (not interpolated) when extrapolating beyond the domain.
+    _, extrap = waveform.get_value(np.array([-1.0, 5.0]))
+    assert list(extrap) == ["ohmic", "nbi"]
+
+
+def test_boolean_waveform():
+    """A waveform of boolean constants evaluates as a zero-order-hold step function."""
+    waveform = Waveform(
+        waveform=[
+            {"user_type": "constant", "user_value": False, "user_duration": 2},
+            {"user_type": "constant", "user_value": True, "user_duration": 2},
+        ]
+    )
+    assert waveform.is_categorical
+
+    _, values = waveform.get_value(np.array([0.0, 1.0, 2.0, 3.0]))
+    assert [bool(v) for v in values] == [False, False, True, True]
+
+
+def test_numeric_waveform_evaluates_to_float():
+    """A numeric waveform evaluates to a float array (the int value is not stepwise)."""
+    waveform = Waveform(
+        waveform=[{"user_type": "constant", "user_value": 8, "user_duration": 3}]
+    )
+    assert not waveform.is_categorical
+    _, values = waveform.get_value(np.array([0.0, 1.0, 2.0, 3.0]))
+    assert values.dtype == float
+
+
+def test_mixing_categorical_and_numeric_is_flagged():
+    """Mixing categorical (string/bool) and numeric tendencies adds an annotation."""
+    waveform = Waveform(
+        waveform=[
+            {"user_type": "constant", "user_value": "nbi", "user_duration": 2},
+            {"user_type": "constant", "user_value": 3, "user_duration": 2},
+        ]
+    )
+    assert waveform.annotations
+    assert any("mix" in a["text"].lower() for a in waveform.annotations)
+
+
+def test_homogeneous_categorical_not_flagged():
+    """A waveform of only categorical values is not flagged as mixed."""
+    waveform = Waveform(
+        waveform=[
+            {"user_type": "constant", "user_value": "nbi", "user_duration": 2},
+            {"user_type": "constant", "user_value": "ec", "user_duration": 2},
+        ]
+    )
+    assert not waveform.annotations
diff --git a/waveform_editor/tendencies/base.py b/waveform_editor/tendencies/base.py
index 50b43198..f92246d3 100644
--- a/waveform_editor/tendencies/base.py
+++ b/waveform_editor/tendencies/base.py
@@ -61,8 +61,8 @@ class BaseTendency(param.Parameterized):
         values from the start value of this tendency.
         """,
     )
-    start_value = param.Number(default=0.0, doc="Value at self.start")
-    end_value = param.Number(default=0.0, doc="Value at self.end")
+    start_value = param.Parameter(default=0.0, doc="Value at self.start")
+    end_value = param.Parameter(default=0.0, doc="Value at self.end")
 
     start_derivative = param.Number(default=0.0, doc="Derivative at self.start")
     end_derivative = param.Number(default=0.0, doc="Derivative at self.end")
@@ -77,6 +77,7 @@ class BaseTendency(param.Parameterized):
     )
     annotations = param.ClassSelector(class_=Annotations, default=Annotations())
     allow_zero_duration = False
+    is_categorical = False
 
     def __init__(self, **kwargs):
         super().__init__()
diff --git a/waveform_editor/tendencies/constant.py b/waveform_editor/tendencies/constant.py
index 6b1cd4af..01c8d7fd 100644
--- a/waveform_editor/tendencies/constant.py
+++ b/waveform_editor/tendencies/constant.py
@@ -10,7 +10,7 @@ class ConstantTendency(BaseTendency):
     Constant tendency class for a constant signal.
     """
 
-    user_value = param.Number(
+    user_value = param.Parameter(
         default=None,
         doc="The constant value of the tendency provided by the user.",
     )
@@ -19,6 +19,13 @@ def __init__(self, **kwargs):
         self.value = 0.0
         super().__init__(**kwargs)
 
+    @property
+    def is_categorical(self):
+        """Whether this constant holds a non-numeric (categorical) value, e.g. a
+        string or boolean, that is held as a step rather than interpolated."""
+        value = self.value
+        return isinstance(value, bool) or not isinstance(value, (int, float))
+
     def get_value(
         self, time: np.ndarray | None = None
     ) -> tuple[np.ndarray, np.ndarray]:
@@ -33,7 +40,7 @@ def get_value(
         """
         if time is None:
             time = np.array([self.start, self.end])
-        values = self.value * np.ones(len(time))
+        values = np.full(len(time), self.value)
         return time, values
 
     def get_derivative(self, time: np.ndarray) -> np.ndarray:
diff --git a/waveform_editor/waveform.py b/waveform_editor/waveform.py
index 17011175..a7d77373 100644
--- a/waveform_editor/waveform.py
+++ b/waveform_editor/waveform.py
@@ -49,6 +49,12 @@ def __init__(
         if waveform is not None:
             self._process_waveform(waveform)
 
+    @property
+    def is_categorical(self):
+        """Whether this waveform produces non-numeric (categorical) values, e.g.
+        strings or booleans, which are held as steps rather than interpolated."""
+        return any(t.is_categorical for t in self.tendencies)
+
     def get_value(
         self, time: np.ndarray | None = None
     ) -> tuple[np.ndarray, np.ndarray]:
@@ -97,7 +103,11 @@ def _evaluate_tendencies(self, time, eval_derivatives=False):
         Returns:
             numpy array containing the computed values.
         """
-        values = np.zeros_like(time, dtype=float)
+        is_categorical = self.is_categorical and not eval_derivatives
+        if is_categorical:
+            values = np.empty(len(time), dtype=object)
+        else:
+            values = np.zeros_like(time, dtype=float)
 
         for i, tendency in enumerate(self.tendencies):
             mask = (time >= tendency.start) & (time <= tendency.end)
@@ -107,17 +117,17 @@ def _evaluate_tendencies(self, time, eval_derivatives=False):
                 else:
                     _, values[mask] = tendency.get_value(time[mask])
 
-            # Handle gaps between tendencies, we linearly interpolate between the
-            # gap values.
+            # Handle gaps between tendencies (hold for strings, interpolate otherwise).
             if i and tendency.prev_tendency.end < tendency.start:
                 prev_tendency = tendency.prev_tendency
                 mask = (time < tendency.start) & (time > prev_tendency.end)
-                slope = (tendency.start_value - prev_tendency.end_value) / (
-                    tendency.start - prev_tendency.end
-                )
                 if np.any(mask):
                     if eval_derivatives:
-                        values[mask] = slope
+                        values[mask] = (
+                            tendency.start_value - prev_tendency.end_value
+                        ) / (tendency.start - prev_tendency.end)
+                    elif is_categorical:
+                        values[mask] = prev_tendency.end_value
                     else:
                         values[mask] = np.interp(
                             time[mask],
@@ -173,11 +183,29 @@ def _process_waveform(self, waveform):
             self.tendencies[i - 1].set_next_tendency(self.tendencies[i])
             self.tendencies[i].set_previous_tendency(self.tendencies[i - 1])
 
+        self._validate_value_types()
+
         self.update_annotations()
 
         for tendency in self.tendencies:
             tendency.param.watch(self.update_annotations, "annotations")
 
+    def _validate_value_types(self):
+        """Categorical (e.g. string or boolean) and numeric tendencies cannot be
+        mixed within a single waveform, as the gaps between them cannot be
+        interpolated. Flag the first tendency whose type breaks the pattern."""
+        if not self.tendencies:
+            return
+        first_is_categorical = self.tendencies[0].is_categorical
+        for tendency in self.tendencies[1:]:
+            if tendency.is_categorical != first_is_categorical:
+                error_msg = (
+                    "Cannot mix categorical (e.g. string or boolean) and numeric "
+                    "values within a single waveform.\n"
+                )
+                self.annotations.add(tendency.line_number, error_msg)
+                break
+
     def update_annotations(self, event=None):
         """Merges the annotations of the individual tendencies into the annotations
         of this waveform."""