matplotlib
diff --git a/‎src/napari_matplotlib/base.py
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+11-4Lines changed: 11 additions & 4 deletions b/‎src/napari_matplotlib/base.py
Copy file name to clipboardExpand all lines: src/napari_matplotlib/base.py
+11-4Lines changed: 11 additions & 4 deletions
diff --git a/‎src/napari_matplotlib/histogram.py
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+4-1Lines changed: 4 additions & 1 deletion b/‎src/napari_matplotlib/histogram.py
Copy file name to clipboardExpand all lines: src/napari_matplotlib/histogram.py
+4-1Lines changed: 4 additions & 1 deletion
diff --git a/‎src/napari_matplotlib/scatter.py
Copy file name to clipboardExpand all lines: src/napari_matplotlib/scatter.py
+3-1Lines changed: 3 additions & 1 deletion b/‎src/napari_matplotlib/scatter.py
Copy file name to clipboardExpand all lines: src/napari_matplotlib/scatter.py
+3-1Lines changed: 3 additions & 1 deletion
diff --git a/‎src/napari_matplotlib/slice.py
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+4-2Lines changed: 4 additions & 2 deletions b/‎src/napari_matplotlib/slice.py
Copy file name to clipboardExpand all lines: src/napari_matplotlib/slice.py
+4-2Lines changed: 4 additions & 2 deletions
diff --git a/‎src/napari_matplotlib/util.py
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+29Lines changed: 29 additions & 0 deletions b/‎src/napari_matplotlib/util.py
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+29Lines changed: 29 additions & 0 deletions
@@ -10,6 +10,8 @@
 from qtpy.QtGui import QIcon
 from qtpy.QtWidgets import QVBoxLayout, QWidget
 
+from .util import Interval
+
 mpl.rc("axes", edgecolor="white")
 mpl.rc("axes", facecolor="#262930")
 mpl.rc("axes", labelcolor="white")
@@ -65,6 +67,11 @@ def __init__(self, napari_viewer: napari.viewer.Viewer):
 
         self.setup_callbacks()
 
+    # Accept any number of input layers by default
+    n_layers_input = Interval(None, None)
+    # Accept any type of input layer by default
+    input_layer_types = (napari.layers.Layer,)
+
     @property
     def n_selected_layers(self) -> int:
         """
@@ -104,10 +111,10 @@ def _draw(self) -> None:
         figure if so.
         """
         self.clear()
-        if self.n_selected_layers != self.n_layers_input:
-            self.canvas.draw()
-            return
-        self.draw()
+        if self.n_selected_layers in self.n_layers_input and all(
+            isinstance(layer, self.input_layer_types) for layer in self.layers
+        ):
+            self.draw()
         self.canvas.draw()
 
     def clear(self) -> None:
 
@@ -6,6 +6,8 @@
 
 import napari
 
+from .util import Interval
+
 _COLORS = {"r": "tab:red", "g": "tab:green", "b": "tab:blue"}
 
 
@@ -14,7 +16,8 @@ class HistogramWidget(NapariMPLWidget):
     Display a histogram of the currently selected layer.
     """
 
-    n_layers_input = 1
+    n_layers_input = Interval(1, 1)
+    input_layer_types = (napari.layers.Image,)
 
     def __init__(self, napari_viewer: napari.viewer.Viewer):
         super().__init__(napari_viewer)
 
@@ -6,6 +6,7 @@
 from magicgui import magicgui
 
 from .base import NapariMPLWidget
+from .util import Interval
 
 __all__ = ["ScatterWidget", "FeaturesScatterWidget"]
 
@@ -84,7 +85,8 @@ class ScatterWidget(ScatterBaseWidget):
     of a scatter plot, to avoid too many scatter points.
     """
 
-    n_layers_input = 2
+    n_layers_input = Interval(2, 2)
+    input_layer_types = (napari.layers.Image,)
 
     def __init__(
         self,
 
@@ -4,7 +4,8 @@
 import numpy as np
 from qtpy.QtWidgets import QComboBox, QHBoxLayout, QLabel, QSpinBox
 
-from napari_matplotlib.base import NapariMPLWidget
+from .base import NapariMPLWidget
+from .util import Interval
 
 __all__ = ["SliceWidget"]
 
@@ -17,7 +18,8 @@ class SliceWidget(NapariMPLWidget):
     Plot a 1D slice along a given dimension.
     """
 
-    n_layers_input = 1
+    n_layers_input = Interval(1, 1)
+    input_layer_types = (napari.layers.Image,)
 
     def __init__(self, napari_viewer: napari.viewer.Viewer):
         # Setup figure/axes
 
@@ -0,0 +1,29 @@
+from typing import Optional
+
+
+class Interval:
+    def __init__(self, lower_bound: Optional[int], upper_bound: Optional[int]):
+        """
+        Parameters
+        ----------
+        lower_bound, upper_bound:
+            Bounds. Use `None` to specify an open bound.
+        """
+        if (
+            lower_bound is not None
+            and upper_bound is not None
+            and lower_bound > upper_bound
+        ):
+            raise ValueError("lower_bound must be <= upper_bound")
+
+        self.lower = lower_bound
+        self.upper = upper_bound
+
+    def __contains__(self, val):
+        if not isinstance(val, int):
+            return NotImplemented
+        if self.lower is not None and val < self.lower:
+            return False
+        if self.upper is not None and val > self.upper:
+            return False
+        return True