matplotlib · anntzer · Oct 10, 2018
diff --git a/doc/users/next_whats_new/2018-10-10-AL.rst b/doc/users/next_whats_new/2018-10-10-AL.rst
@@ -0,0 +1,8 @@
+Improved formatting of image values under cursor when a colorbar is present
+```````````````````````````````````````````````````````````````````````````
+
+To format image values under the mouse cursor into the status bar, we now
+use the image's colorbar formatter (creating a hidden colorbar 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 ``10000``
+and ``10001``), whereas both values were previously displayed as ``1e+04``.
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -936,14 +936,19 @@ def get_cursor_data(self, event):
            return arr[i, j]

    def format_cursor_data(self, data):
-        if np.ndim(data) == 0 and self.colorbar:
-            return (
-                "["
+        if np.ndim(data):
+            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.
+            ax.yaxis._update_ticks()
+        return ("["
                + cbook.strip_math(
                    self.colorbar.formatter.format_data_short(data)).strip()
                + "]")
-        else:
-            return super().format_cursor_data(data)


 class NonUniformImage(AxesImage):

diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -288,11 +288,11 @@ def test_cursor_data():


 @pytest.mark.parametrize(
-    "data, text_without_colorbar, text_with_colorbar", [
-        ([[10001, 10000]], "[1e+04]", "[10001]"),
-        ([[.123, .987]], "[0.123]", "[0.123]"),
+    "data, text", [
+        ([[10001, 10000]], "[10001]"),
+        ([[.123, .987]], "[0.123]"),
 ])
-def test_format_cursor_data(data, text_without_colorbar, text_with_colorbar):
+def test_format_cursor_data(data, text):
    from matplotlib.backend_bases import MouseEvent

    fig, ax = plt.subplots()
@@ -302,14 +302,7 @@ def test_format_cursor_data(data, text_without_colorbar, text_with_colorbar):
    event = MouseEvent('motion_notify_event', fig.canvas, xdisp, ydisp)
    assert im.get_cursor_data(event) == data[0][0]
    assert im.format_cursor_data(im.get_cursor_data(event)) \
-        == text_without_colorbar
-
-    fig.colorbar(im)
-    fig.canvas.draw()  # This is necessary to set up the colorbar formatter.
-
-    assert im.get_cursor_data(event) == data[0][0]
-    assert im.format_cursor_data(im.get_cursor_data(event)) \
-        == text_with_colorbar
+        == text


 @image_comparison(baseline_images=['image_clip'], style='mpl20')