BUG: Plotting use FixedLocator

For string x values or multiindex, the axis locator is now set to a
FixedLocator for Line and Area plot. The ticklabel stay consistent
when scaling and zooming.
This impact the following issues:
GH7612, GH15912, GH22334

Author: nrebena

pandas/plotting/_core.py

@@ -410,8 +410,13 @@ def get_label(i):
 
         if self.orientation == 'vertical' or self.orientation is None:
             if self._need_to_set_index:
-                xticklabels = [get_label(x) for x in ax.get_xticks()]
+                xticks = ax.get_xticks()
+                xticklabels = [get_label(x) for x in xticks]
                 ax.set_xticklabels(xticklabels)
+                from matplotlib.ticker import FixedLocator, FixedFormatter
+                ax.xaxis.set_major_locator(FixedLocator(xticks))
+                ax.xaxis.set_major_formatter(FixedFormatter(xticklabels))
+
             self._apply_axis_properties(ax.xaxis, rot=self.rot,
                                         fontsize=self.fontsize)
             self._apply_axis_properties(ax.yaxis, fontsize=self.fontsize)
@@ -422,8 +427,12 @@ def get_label(i):
 
         elif self.orientation == 'horizontal':
             if self._need_to_set_index:
-                yticklabels = [get_label(y) for y in ax.get_yticks()]
+                yticks = ax.get_yticks()
+                yticklabels = [get_label(y) for y in yticks]
                 ax.set_yticklabels(yticklabels)
+                from matplotlib.ticker import FixedLocator, FixedFormatter
+                ax.xaxis.set_major_locator(FixedLocator(yticks))
+                ax.xaxis.set_major_formatter(FixedFormatter(yticklabels))
             self._apply_axis_properties(ax.yaxis, rot=self.rot,
                                         fontsize=self.fontsize)
             self._apply_axis_properties(ax.xaxis, fontsize=self.fontsize)