Offset text position (matplotlib#15592)

* Rewrite tests so they don't require saved png files

* Add internal flag _text_offset_position, use it to update offset text position

* Remove unnecessary .set_va

* Fix style (as per review), as default behaviour

* Add whatsnew entry

Author: MarcoGorelli

doc/users/next_whats_new/2019-11-13-offset-text-position.rst

@@ -0,0 +1,4 @@
+Offset text is now set to the top when using axis.tick_top()
+------------------------------------------------------------
+
+Solves the issue that the power indicator (e.g. 1e4) stayed on the bottom, even if the ticks were on the top.

lib/matplotlib/axis.py

@@ -2060,14 +2060,27 @@ def _update_offset_text_position(self, bboxes, bboxes2):
         boxes of all the ticklabels
         """
         x, y = self.offsetText.get_position()
-        if not len(bboxes):
-            bottom = self.axes.bbox.ymin
+        if not hasattr(self, '_tick_position'):
+            self._tick_position = 'bottom'
+        if self._tick_position == 'bottom':
+            if not len(bboxes):
+                bottom = self.axes.bbox.ymin
+            else:
+                bbox = mtransforms.Bbox.union(bboxes)
+                bottom = bbox.y0
+            self.offsetText.set_position(
+                (x, bottom - self.OFFSETTEXTPAD * self.figure.dpi / 72)
+            )
         else:
-            bbox = mtransforms.Bbox.union(bboxes)
-            bottom = bbox.y0
-        self.offsetText.set_position(
-            (x, bottom - self.OFFSETTEXTPAD * self.figure.dpi / 72)
-        )
+            if not len(bboxes2):
+                top = self.axes.bbox.ymax
+            else:
+                bbox = mtransforms.Bbox.union(bboxes2)
+                top = bbox.y1
+            self.offsetText.set_va('top')
+            self.offsetText.set_position(
+                (x, top + self.OFFSETTEXTPAD * self.figure.dpi / 72)
+            )
 
     def get_text_heights(self, renderer):
         """
@@ -2107,9 +2120,11 @@ def set_ticks_position(self, position):
         if position == 'top':
             self.set_tick_params(which='both', top=True, labeltop=True,
                                  bottom=False, labelbottom=False)
+            self._tick_position = 'top'
         elif position == 'bottom':
             self.set_tick_params(which='both', top=False, labeltop=False,
                                  bottom=True, labelbottom=True)
+            self._tick_position = 'bottom'
         elif position == 'both':
             self.set_tick_params(which='both', top=True,
                                  bottom=True)
@@ -2119,6 +2134,7 @@ def set_ticks_position(self, position):
         elif position == 'default':
             self.set_tick_params(which='both', top=True, labeltop=False,
                                  bottom=True, labelbottom=True)
+            self._tick_position = 'bottom'
         else:
             raise ValueError("invalid position: %s" % position)
         self.stale = True

lib/matplotlib/tests/test_axes.py

@@ -5201,10 +5201,24 @@ def test_unicode_hist_label():
 
 def test_move_offsetlabel():
     data = np.random.random(10) * 1e-22
+
     fig, ax = plt.subplots()
     ax.plot(data)
+    fig.canvas.draw()
+    before = ax.yaxis.offsetText.get_position()
     ax.yaxis.tick_right()
-    assert (1, 0.5) == ax.yaxis.offsetText.get_position()
+    fig.canvas.draw()
+    after = ax.yaxis.offsetText.get_position()
+    assert after[0] > before[0] and after[1] == before[1]
+
+    fig, ax = plt.subplots()
+    ax.plot(data)
+    fig.canvas.draw()
+    before = ax.xaxis.offsetText.get_position()
+    ax.xaxis.tick_top()
+    fig.canvas.draw()
+    after = ax.xaxis.offsetText.get_position()
+    assert after[0] == before[0] and after[1] > before[1]
 
 
 @image_comparison(['rc_spines.png'], savefig_kwarg={'dpi': 40})