BUG: Plot with label would overwrite index name

doc/source/whatsnew/v0.15.0.txt

@@ -1149,4 +1149,3 @@ Bug Fixes
 - Suppress FutureWarning generated by NumPy when comparing object arrays containing NaN for equality (:issue:`7065`)
 - Bug in ``DataFrame.eval()`` where the dtype of the ``not`` operator (``~``)
   was not correctly inferred as ``bool``.
-

doc/source/whatsnew/v0.15.1.txt

@@ -138,3 +138,8 @@ Bug Fixes
 - Fix ``shape`` attribute for ``MultiIndex`` (:issue:`8609`)
 - Bug in ``GroupBy`` where a name conflict between the grouper and columns
   would break ``groupby`` operations (:issue:`7115`, :issue:`8112`)
+
+
+
+- Fixed a bug where plotting a column ``y`` and specifying a label
+would mutate the index name of the DataFrame ``y`` came from (:issue:`8494`)

pandas/tests/test_graphics.py

@@ -1058,6 +1058,14 @@ def test_explicit_label(self):
         ax = df.plot(x='a', y='b', label='LABEL')
         self._check_text_labels(ax.xaxis.get_label(), 'LABEL')
 
+    @slow
+    def test_donot_overwrite_index_name(self):
+        # GH 8494
+        df = DataFrame(randn(2, 2), columns=['a', 'b'])
+        df.index.name = 'NAME'
+        df.plot(y='b', label='LABEL')
+        self.assertEqual(df.index.name, 'NAME')
+
     @slow
     def test_plot_xy(self):
         # columns.inferred_type == 'string'

pandas/tools/plotting.py

@@ -2261,7 +2261,8 @@ def _plot(data, x=None, y=None, subplots=False,
             elif y is not None:
                 if com.is_integer(y) and not data.columns.holds_integer():
                     y = data.columns[y]
-                data = data[y]  # converted to series actually
+                # converted to series actually. copy to not modify
+                data = data[y].copy()
                 data.index.name = y
         plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
     else:
@@ -2276,7 +2277,7 @@ def _plot(data, x=None, y=None, subplots=False,
                     y = data.columns[y]
                 label = x if x is not None else data.index.name
                 label = kwds.pop('label', label)
-                series = data[y]
+                series = data[y].copy()  # Don't modify
                 series.index.name = label
 
                 for kw in ['xerr', 'yerr']: