Create a hidden colorbar on-the-fly to format imshow cursor data.

anntzer · anntzer · commit f33913f73619 · 2018-10-10T13:51:13.000+02:00
... if no colorbar exists yet.
diff --git a/doc/users/next_whats_new/2018-10-10-AL.rst b/doc/users/next_whats_new/2018-10-10-AL.rst
@@ -1,9 +1,10 @@
 Improved formatting of image values under cursor when a colorbar is present
 ```````````````````````````````````````````````````````````````````````````
 
-When a colorbar is present, its formatter is now used to format the image
-values under the mouse cursor in the status bar.  For example, for an image
-displaying the values 10,000 and 10,001, the statusbar will now (using default
-settings) display the values as ``0.0+1e4`` and ``1.0+1e4`` (or ``10000.0``
-and ``10001.0`` if the offset-text is disabled on the colorbar), whereas both
-values were previously displayed as ``1e+04``.
+To format image values under the mouse cursor into the status bar, we now
+use the image's colorbar formatter (creating a hidden one on-the-fly if
+necessary).  For example, for an image displaying the values 10,000 and
+10,001, the statusbar will now (using default settings) display the values as
+``0.0+1e4`` and ``1.0+1e4`` (or ``10000.0`` and ``10001.0`` if the offset-text
+is disabled on colorbars), whereas both values were previously displayed as
+``1e+04``.
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -908,13 +908,17 @@ def get_cursor_data(self, event):
             return arr[i, j]
 
     def format_cursor_data(self, data):
-        if self.colorbar:
-            return ("["
-                    + cbook.strip_math(self.colorbar.formatter(data))
-                    + cbook.strip_math(self.colorbar.formatter.get_offset())
-                    + "]")
-        else:
-            return super().format_cursor_data(data)
+        if not self.colorbar:
+            from matplotlib.figure import Figure
+            fig = Figure()
+            ax = fig.subplots()
+            self.colorbar = fig.colorbar(self, cax=ax)
+            # This will call the locator and call set_locs() on the formatter.
+            list(ax.yaxis.iter_ticks())
+        return ("["
+                + cbook.strip_math(self.colorbar.formatter(data))
+                + cbook.strip_math(self.colorbar.formatter.get_offset())
+                + "]")
 
 
 class NonUniformImage(AxesImage):
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -270,12 +270,6 @@ def test_format_cursor_data():
 
     xdisp, ydisp = ax.transData.transform_point([0, 0])
     event = MouseEvent('motion_notify_event', fig.canvas, xdisp, ydisp)
-    assert im.get_cursor_data(event) == 10000
-    assert im.format_cursor_data(im.get_cursor_data(event)) == "[1e+04]"
-
-    fig.colorbar(im)
-    fig.canvas.draw()  # This is necessary to set up the colorbar formatter.
-
     assert im.get_cursor_data(event) == 10000
     assert im.format_cursor_data(im.get_cursor_data(event)) == "[0.0+1e4]"
 
