Don't try to use the colorbar formatter to format RGBA data.

anntzer · anntzer · commit 17420ec0cb10 · 2019-04-28T21:04:35.000+02:00
A colorbar doesn't make much sense for RGBA data, but matplotlib does
allow one to be constructed; when formatting the cursor data for a RGBA
image we must be careful to not use the colorbar formatter.

Otherwise, after `plt.imshow(np.random.rand(5, 5, 3)); plt.colorbar()`,
when moving the mouse cursor over the image, we get
```
&lt;elided&gt;
  File "/usr/lib/python3.7/site-packages/matplotlib/ticker.py", line 604, in format_data_short
    return '%-12g' % value
  File "/usr/lib/python3.7/site-packages/numpy/ma/core.py", line 4296, in __float__
    raise TypeError("Only length-1 arrays can be converted "
TypeError: Only length-1 arrays can be converted to Python scalars
```
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -930,7 +930,7 @@ def get_cursor_data(self, event):
             return arr[i, j]
 
     def format_cursor_data(self, data):
-        if self.colorbar:
+        if np.ndim(data) == 0 and self.colorbar:
             return (
                 "["
                 + cbook.strip_math(
