Don't read full cube into mem for histogramming

src/napari_matplotlib/histogram.py

@@ -33,14 +33,21 @@ def draw(self) -> None:
         Clear the axes and histogram the currently selected layer/slice.
         """
         layer = self.layers[0]
-        bins = np.linspace(np.min(layer.data), np.max(layer.data), 100)
+        print(layer.data.shape)
 
         if layer.data.ndim - layer.rgb == 3:
             # 3D data, can be single channel or RGB
             data = layer.data[self.current_z]
             self.axes.set_title(f"z={self.current_z}")
         else:
             data = layer.data
+        # Read data into memory if it's a dask array
+        data = np.asarray(data)
+
+        # Important to calculate bins after slicing 3D data, to avoid reading
+        # whole cube into memory.
+        bins = np.linspace(np.min(data), np.max(data), 100)
+        print(bins)
 
         if layer.rgb:
             # Histogram RGB channels independently