BUG: secondary_y may not show legend properly

doc/source/whatsnew/v0.16.1.txt

@@ -75,7 +75,7 @@ Bug Fixes
 - Bug in ``DataFrame`` slicing may not retain metadata (:issue:`9776`)
 - Bug where ``TimdeltaIndex`` were not properly serialized in fixed ``HDFStore`` (:issue:`9635`)
 
-
+- Bug in plotting continuously using ``secondary_y`` may not show legend properly. (:issue:`9610`, :issue:`9779`)
 
 
 - Bug in ``Series.quantile`` on empty Series of type ``Datetime`` or ``Timedelta`` (:issue:`9675`)

pandas/tests/test_graphics.py

@@ -755,6 +755,101 @@ def test_hist_no_overlap(self):
         axes = fig.get_axes()
         self.assertEqual(len(axes), 2)
 
+    @slow
+    def test_hist_secondary_legend(self):
+        # GH 9610
+        df = DataFrame(np.random.randn(30, 4), columns=list('abcd'))
+
+        # primary -> secondary
+        ax = df['a'].plot(kind='hist', legend=True)
+        df['b'].plot(kind='hist', ax=ax, legend=True, secondary_y=True)
+        # both legends are dran on left ax
+        # left and right axis must be visible
+        self._check_legend_labels(ax, labels=['a', 'b (right)'])
+        self.assertTrue(ax.get_yaxis().get_visible())
+        self.assertTrue(ax.right_ax.get_yaxis().get_visible())
+        tm.close()
+
+        # secondary -> secondary
+        ax = df['a'].plot(kind='hist', legend=True, secondary_y=True)
+        df['b'].plot(kind='hist', ax=ax, legend=True, secondary_y=True)
+        # both legends are draw on left ax
+        # left axis must be invisible, right axis must be visible
+        self._check_legend_labels(ax.left_ax, labels=['a (right)', 'b (right)'])
+        self.assertFalse(ax.left_ax.get_yaxis().get_visible())
+        self.assertTrue(ax.get_yaxis().get_visible())
+        tm.close()
+
+        # secondary -> primary
+        ax = df['a'].plot(kind='hist', legend=True, secondary_y=True)
+        # right axes is returned
+        df['b'].plot(kind='hist', ax=ax, legend=True)
+        # both legends are draw on left ax
+        # left and right axis must be visible
+        self._check_legend_labels(ax.left_ax, labels=['a (right)', 'b'])
+        self.assertTrue(ax.left_ax.get_yaxis().get_visible())
+        self.assertTrue(ax.get_yaxis().get_visible())
+        tm.close()
+
+    @slow
+    def test_df_series_secondary_legend(self):
+        # GH 9779
+        df = DataFrame(np.random.randn(30, 3), columns=list('abc'))
+        s = Series(np.random.randn(30), name='x')
+
+        # primary -> secondary (without passing ax)
+        ax = df.plot()
+        s.plot(legend=True, secondary_y=True)
+        # both legends are dran on left ax
+        # left and right axis must be visible
+        self._check_legend_labels(ax, labels=['a', 'b', 'c', 'x (right)'])
+        self.assertTrue(ax.get_yaxis().get_visible())
+        self.assertTrue(ax.right_ax.get_yaxis().get_visible())
+        tm.close()
+
+        # primary -> secondary (with passing ax)
+        ax = df.plot()
+        s.plot(ax=ax, legend=True, secondary_y=True)
+        # both legends are dran on left ax
+        # left and right axis must be visible
+        self._check_legend_labels(ax, labels=['a', 'b', 'c', 'x (right)'])
+        self.assertTrue(ax.get_yaxis().get_visible())
+        self.assertTrue(ax.right_ax.get_yaxis().get_visible())
+        tm.close()
+
+        # seconcary -> secondary (without passing ax)
+        ax = df.plot(secondary_y=True)
+        s.plot(legend=True, secondary_y=True)
+        # both legends are dran on left ax
+        # left axis must be invisible and right axis must be visible
+        expected = ['a (right)', 'b (right)', 'c (right)', 'x (right)']
+        self._check_legend_labels(ax.left_ax, labels=expected)
+        self.assertFalse(ax.left_ax.get_yaxis().get_visible())
+        self.assertTrue(ax.get_yaxis().get_visible())
+        tm.close()
+
+        # secondary -> secondary (with passing ax)
+        ax = df.plot(secondary_y=True)
+        s.plot(ax=ax, legend=True, secondary_y=True)
+        # both legends are dran on left ax
+        # left axis must be invisible and right axis must be visible
+        expected = ['a (right)', 'b (right)', 'c (right)', 'x (right)']
+        self._check_legend_labels(ax.left_ax, expected)
+        self.assertFalse(ax.left_ax.get_yaxis().get_visible())
+        self.assertTrue(ax.get_yaxis().get_visible())
+        tm.close()
+
+        # secondary -> secondary (with passing ax)
+        ax = df.plot(secondary_y=True, mark_right=False)
+        s.plot(ax=ax, legend=True, secondary_y=True)
+        # both legends are dran on left ax
+        # left axis must be invisible and right axis must be visible
+        expected = ['a', 'b', 'c', 'x (right)']
+        self._check_legend_labels(ax.left_ax, expected)
+        self.assertFalse(ax.left_ax.get_yaxis().get_visible())
+        self.assertTrue(ax.get_yaxis().get_visible())
+        tm.close()
+
     @slow
     def test_plot_fails_with_dupe_color_and_style(self):
         x = Series(randn(2))

pandas/tools/plotting.py

@@ -926,7 +926,19 @@ def generate(self):
     def _args_adjust(self):
         pass
 
-    def _maybe_right_yaxis(self, ax):
+    def _has_plotted_object(self, ax):
+        """check whether ax has data"""
+        return (len(ax.lines) != 0 or
+                len(ax.artists) != 0 or
+                len(ax.containers) != 0)
+
+    def _maybe_right_yaxis(self, ax, axes_num):
+        if not self.on_right(axes_num):
+            if hasattr(ax, 'left_ax'):
+                # secondary axes may be passed as axes
+                return ax.left_ax
+            return ax
+
         if hasattr(ax, 'right_ax'):
             return ax.right_ax
         else:
@@ -936,7 +948,7 @@ def _maybe_right_yaxis(self, ax):
             orig_ax.right_ax, new_ax.left_ax = new_ax, orig_ax
             new_ax.right_ax = new_ax
 
-            if len(orig_ax.get_lines()) == 0:  # no data on left y
+            if not self._has_plotted_object(orig_ax):  # no data on left y
                 orig_ax.get_yaxis().set_visible(False)
             return new_ax
 
@@ -978,7 +990,15 @@ def result(self):
             else:
                 return self.axes
         else:
-            return self.axes[0]
+            sec_true = isinstance(self.secondary_y, bool) and self.secondary_y
+            all_sec = (com.is_list_like(self.secondary_y) and
+                       len(self.secondary_y) == self.nseries)
+            if (sec_true or all_sec):
+                # if all data is plotted on secondary,
+                # return secondary axes
+                return self.axes[0].right_ax
+            else:
+                return self.axes[0]
 
     def _compute_plot_data(self):
         numeric_data = self.data.convert_objects()._get_numeric_data()
@@ -1128,8 +1148,8 @@ def _make_legend(self):
 
     def _get_ax_legend(self, ax):
         leg = ax.get_legend()
-        other_ax = (getattr(ax, 'right_ax', None) or
-                    getattr(ax, 'left_ax', None))
+        other_ax = (getattr(ax, 'left_ax', None) or
+                    getattr(ax, 'right_ax', None))
         other_leg = None
         if other_ax is not None:
             other_leg = other_ax.get_legend()
@@ -1221,20 +1241,11 @@ def _get_ax(self, i):
         if self.subplots:
             ax = self.axes[i]
 
-            if self.on_right(i):
-                ax = self._maybe_right_yaxis(ax)
-                self.axes[i] = ax
+            ax = self._maybe_right_yaxis(ax, i)
+            self.axes[i] = ax
         else:
             ax = self.axes[0]
-
-            if self.on_right(i):
-                ax = self._maybe_right_yaxis(ax)
-
-                sec_true = isinstance(self.secondary_y, bool) and self.secondary_y
-                all_sec = (com.is_list_like(self.secondary_y) and
-                           len(self.secondary_y) == self.nseries)
-                if sec_true or all_sec:
-                    self.axes[0] = ax
+            ax = self._maybe_right_yaxis(ax, i)
 
         ax.get_yaxis().set_visible(True)
         return ax
@@ -1971,7 +1982,7 @@ def _make_plot(self):
                 kwds['style'] = style
 
             artists = plotf(ax, y, column_num=i, **kwds)
-            self._add_legend_handle(artists[0], label)
+            self._add_legend_handle(artists[0], label, index=i)
 
     def _post_plot_logic(self):
         if self.orientation == 'horizontal':