BUG: mixed freq timeseries plotting with shared axes (GH13341)

Author: jorisvandenbossche

doc/source/whatsnew/v0.19.0.txt

@@ -1584,3 +1584,5 @@ Bug Fixes
 - ``PeridIndex`` can now accept ``list`` and ``array`` which contains ``pd.NaT`` (:issue:`13430`)
 - Bug in ``df.groupby`` where ``.median()`` returns arbitrary values if grouped dataframe contains empty bins (:issue:`13629`)
 - Bug in ``Index.copy()`` where ``name`` parameter was ignored (:issue:`14302`)
+- Bug in plotting regular and irregular timeseries using shared axes
+  (``sharex=True`` or ``ax.twinx()``) (:issue:`13341`, :issue:`14322`).

pandas/tests/plotting/test_datetimelike.py

@@ -778,6 +778,42 @@ def test_mixed_freq_irreg_period(self):
         irreg.plot()
         ps.plot()
 
+    def test_mixed_freq_shared_ax(self):
+        import matplotlib.pyplot as plt
+
+        # GH13341, using sharex=True
+        idx1 = date_range('2015-01-01', periods=3, freq='M')
+        idx2 = idx1[:1].union(idx1[2:])
+        s1 = Series(range(len(idx1)), idx1)
+        s2 = Series(range(len(idx2)), idx2)
+
+        fig, (ax1, ax2) = plt.subplots(nrows=2, sharex=True)
+        s1.plot(ax=ax1)
+        s2.plot(ax=ax2)
+
+        self.assertEqual(ax1.freq, 'M')
+        self.assertEqual(ax2.freq, 'M')
+        self.assertEqual(ax1.lines[0].get_xydata()[0, 0],
+                         ax2.lines[0].get_xydata()[0, 0])
+
+        # using twinx
+        fig, ax1 = plt.subplots()
+        ax2 = ax1.twinx()
+        s1.plot(ax=ax1)
+        s2.plot(ax=ax2)
+
+        self.assertEqual(ax1.lines[0].get_xydata()[0, 0],
+                         ax2.lines[0].get_xydata()[0, 0])
+
+        # TODO
+        # plotting the irregular first does not yet work
+        # fig, ax1 = plt.subplots()
+        # ax2 = ax1.twinx()
+        # s2.plot(ax=ax1)
+        # s1.plot(ax=ax2)
+        # self.assertEqual(ax1.lines[0].get_xydata()[0, 0],
+        #                  ax2.lines[0].get_xydata()[0, 0])
+
     @slow
     def test_to_weekly_resampling(self):
         idxh = date_range('1/1/1999', periods=52, freq='W')

pandas/tseries/plotting.py

@@ -162,18 +162,37 @@ def _decorate_axes(ax, freq, kwargs):
     ax.date_axis_info = None
 
 
-def _get_freq(ax, series):
-    # get frequency from data
-    freq = getattr(series.index, 'freq', None)
-    if freq is None:
-        freq = getattr(series.index, 'inferred_freq', None)
-
+def _get_ax_freq(ax):
+    """
+    Get the freq attribute of the ax object if set.
+    Also checks shared axes (eg when using secondary yaxis, sharex=True
+    or twinx)
+    """
     ax_freq = getattr(ax, 'freq', None)
     if ax_freq is None:
+        # check for left/right ax in case of secondary yaxis
         if hasattr(ax, 'left_ax'):
             ax_freq = getattr(ax.left_ax, 'freq', None)
         elif hasattr(ax, 'right_ax'):
             ax_freq = getattr(ax.right_ax, 'freq', None)
+    if ax_freq is None:
+        # check if a shared ax (sharex/twinx) has already freq set
+        shared_axes = ax.get_shared_x_axes().get_siblings(ax)
+        if len(shared_axes) > 1:
+            for shared_ax in shared_axes:
+                ax_freq = getattr(shared_ax, 'freq', None)
+                if ax_freq is not None:
+                    break
+    return ax_freq
+
+
+def _get_freq(ax, series):
+    # get frequency from data
+    freq = getattr(series.index, 'freq', None)
+    if freq is None:
+        freq = getattr(series.index, 'inferred_freq', None)
+
+    ax_freq = _get_ax_freq(ax)
 
     # use axes freq if no data freq
     if freq is None:
@@ -191,7 +210,7 @@ def _get_freq(ax, series):
 
 def _use_dynamic_x(ax, data):
     freq = _get_index_freq(data)
-    ax_freq = getattr(ax, 'freq', None)
+    ax_freq = _get_ax_freq(ax)
 
     if freq is None:  # convert irregular if axes has freq info
         freq = ax_freq
@@ -244,7 +263,7 @@ def _maybe_convert_index(ax, data):
             freq = freq.rule_code
 
         if freq is None:
-            freq = getattr(ax, 'freq', None)
+            freq = _get_ax_freq(ax)
 
         if freq is None:
             raise ValueError('Could not get frequency alias for plotting')