Add some explanation text to top of widgets

docs/changelog.rst

@@ -8,6 +8,8 @@ New features
 ~~~~~~~~~~~~
 - Added ``MPLWidget`` as a widget containing just a Matplotlib canvas
   without any association with a napari viewer.
+- Added text to each widget indicating how many layers need to be selected
+  for the widget to plot something.
 
 Visual improvements
 ~~~~~~~~~~~~~~~~~~~

src/napari_matplotlib/base.py

@@ -10,7 +10,7 @@
 )
 from matplotlib.figure import Figure
 from qtpy.QtGui import QIcon
-from qtpy.QtWidgets import QVBoxLayout, QWidget
+from qtpy.QtWidgets import QLabel, QVBoxLayout, QWidget
 
 from .util import Interval, from_napari_css_get_size_of
 
@@ -133,6 +133,11 @@ class NapariMPLWidget(BaseNapariMPLWidget):
         for creating and working with the Matplotlib figure and any axes.
     """
 
+    #: Number of layers taken as input
+    n_layers_input = Interval(None, None)
+    #: Type of layer taken as input
+    input_layer_types: Tuple[napari.layers.Layer, ...] = (napari.layers.Layer,)
+
     def __init__(
         self,
         napari_viewer: napari.viewer.Viewer,
@@ -144,10 +149,9 @@ def __init__(
         self._setup_callbacks()
         self.layers: List[napari.layers.Layer] = []
 
-    #: Number of layers taken as input
-    n_layers_input = Interval(None, None)
-    #: Type of layer taken as input
-    input_layer_types: Tuple[napari.layers.Layer, ...] = (napari.layers.Layer,)
+        helper_text = self.n_layers_input._helper_text
+        if helper_text is not None:
+            self.layout().insertWidget(0, QLabel(helper_text))
 
     @property
     def n_selected_layers(self) -> int:

src/napari_matplotlib/tests/test_util.py

@@ -16,6 +16,23 @@ def test_interval():
         "string" in interval  # type: ignore
 
 
+@pytest.mark.parametrize(
+    "lower, upper, text",
+    [
+        (None, None, None),
+        (1, None, "Select at least 1 layer to generate plot"),
+        (4, None, "Select at least 4 layers to generate plot"),
+        (None, 1, "Select at most 1 layer to generate plot"),
+        (None, 5939, "Select at most 5939 layers to generate plot"),
+        (1, 1, "Select 1 layer to generate plot"),
+        (2, 2, "Select 2 layers to generate plot"),
+        (1, 2, "Select between 1 and 2 layers to generate plot"),
+    ],
+)
+def test_interval_helper_text(lower, upper, text):
+    assert Interval(lower, upper)._helper_text == text
+
+
 def test_get_size_from_css(mocker):
     """Test getting the max-width and max-height from something in css"""
     test_css = """

src/napari_matplotlib/util.py

@@ -46,6 +46,35 @@ def __contains__(self, val: int) -> bool:
             return False
         return True
 
+    @property
+    def _helper_text(self) -> Optional[str]:
+        """
+        Helper text for widgets.
+        """
+        if self.lower is None and self.upper is None:
+            helper_text = None
+        elif self.lower is not None and self.upper is None:
+            helper_text = (
+                f"Select at least {self.lower} layers to generate plot"
+            )
+        elif self.lower is None and self.upper is not None:
+            helper_text = (
+                f"Select at most {self.upper} layers to generate plot"
+            )
+        elif self.lower == self.upper:
+            helper_text = f"Select {self.lower} layers to generate plot"
+
+        else:
+            helper_text = (
+                f"Select between {self.lower} and "
+                f"{self.upper} layers to generate plot"
+            )
+
+        if helper_text is not None:
+            helper_text = helper_text.replace("1 layers", "1 layer")
+
+        return helper_text
+
 
 def _has_id(nodes: List[tinycss2.ast.Node], id_name: str) -> bool:
     """