Convert axis limit units in Qt plot options widget

Previously axis limits were converted to floats regardless of the actual
value type in Qt backend's plot options widget. This commit makes datetime
axis limits get converted to actual datetime values which are
considerably easier to edit.

Also, the methods used to convert the datetime values back have different
names under PySide2 and PyQt. Previously the options widget was using the
methods provided by PyQt only. We now support both Qt bindings.

Re matplotlib#19075

Author: soininen

lib/matplotlib/backends/qt_editor/_formlayout.py

@@ -342,9 +342,17 @@ def get(self):
             elif isinstance(value, Real):
                 value = float(str(field.text()))
             elif isinstance(value, datetime.datetime):
-                value = field.dateTime().toPyDateTime()
+                datetime_ = field.dateTime()
+                if hasattr(datetime_, "toPyDateTime"):
+                    value = datetime_.toPyDateTime()
+                else:
+                    value = datetime_.toPython()
             elif isinstance(value, datetime.date):
-                value = field.date().toPyDate()
+                date_ = field.date()
+                if hasattr(date_, "toPyDate"):
+                    value = date_.toPyDate()
+                else:
+                    value = date_.toPython()
             else:
                 value = eval(str(field.text()))
             valuelist.append(value)

lib/matplotlib/backends/qt_editor/figureoptions.py

@@ -10,7 +10,7 @@
 from matplotlib import cbook, cm, colors as mcolors, markers, image as mimage
 from matplotlib.backends.qt_compat import QtGui
 from matplotlib.backends.qt_editor import _formlayout
-
+from matplotlib.dates import DateConverter, num2date
 
 LINESTYLES = {'-': 'Solid',
               '--': 'Dashed',
@@ -33,9 +33,17 @@ def figure_edit(axes, parent=None):
     sep = (None, None)  # separator
 
     # Get / General
-    # Cast to builtin floats as they have nicer reprs.
-    xmin, xmax = map(float, axes.get_xlim())
-    ymin, ymax = map(float, axes.get_ylim())
+    def convert_limits(lim, converter):
+        """Convert axis limits for correct input editors."""
+        if isinstance(converter, DateConverter):
+            return map(num2date, lim)
+        # Cast to builtin floats as they have nicer reprs.
+        return map(float, lim)
+
+    xconverter = axes.xaxis.converter
+    xmin, xmax = convert_limits(axes.get_xlim(), xconverter)
+    yconverter = axes.yaxis.converter
+    ymin, ymax = convert_limits(axes.get_ylim(), yconverter)
     general = [('Title', axes.get_title()),
                sep,
                (None, "<b>X-Axis</b>"),
@@ -52,8 +60,6 @@ def figure_edit(axes, parent=None):
                ]
 
     # Save the unit data
-    xconverter = axes.xaxis.converter
-    yconverter = axes.yaxis.converter
     xunits = axes.xaxis.get_units()
     yunits = axes.yaxis.get_units()