API & BUG: allow list-like y argument to df.plot & fix integer arg to x,y

doc/source/whatsnew/v0.23.0.txt

@@ -986,6 +986,7 @@ Plotting
 ^^^^^^^^
 
 - :func:`DataFrame.plot` now raises a ``ValueError`` when the ``x`` or ``y`` argument is improperly formed (:issue:`18671`)
+- Bug in :func:`DataFrame.plot` when ``x`` and ``y`` arguments given as positions caused incorrect referenced columns for line, bar and area plots (:issue:`20056`)
 - Bug in formatting tick labels with ``datetime.time()`` and fractional seconds (:issue:`18478`).
 - :meth:`Series.plot.kde` has exposed the args ``ind`` and ``bw_method`` in the docstring (:issue:`18461`). The argument ``ind`` may now also be an integer (number of sample points).
 
@@ -1041,3 +1042,4 @@ Other
 
 - Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)
 - Bug in accessing a :func:`pandas.get_option`, which raised ``KeyError`` rather than ``OptionError`` when looking up a non-existant option key in some cases (:issue:`19789`)
+- :func:`DataFrame.plot` now supports multiple columns to the ``y`` argument (:issue:`19699`)

pandas/plotting/_core.py

@@ -1751,21 +1751,36 @@ def _plot(data, x=None, y=None, subplots=False,
         plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
     else:
         if isinstance(data, ABCDataFrame):
+            data_cols = data.columns
             if x is not None:
                 if is_integer(x) and not data.columns.holds_integer():
-                    x = data.columns[x]
+                    x = data_cols[x]
                 elif not isinstance(data[x], ABCSeries):
                     raise ValueError("x must be a label or position")
                 data = data.set_index(x)
 
             if y is not None:
-                if is_integer(y) and not data.columns.holds_integer():
-                    y = data.columns[y]
-                elif not isinstance(data[y], ABCSeries):
-                    raise ValueError("y must be a label or position")
-                label = kwds['label'] if 'label' in kwds else y
-                series = data[y].copy()  # Don't modify
-                series.name = label
+                int_y = is_integer(y) or all(is_integer(c) for c in y)
+                if int_y and not data.columns.holds_integer():
+                    y = data_cols[y]
+                elif not isinstance(data[y], (ABCSeries, ABCDataFrame)):
+                    raise ValueError(
+                        "y must be a label or position or list of them"
+                    )
+                label_kw = kwds['label'] if 'label' in kwds else False
+                new_data = data[y].copy()
+
+                if isinstance(data[y], ABCSeries):
+                    label_name = label_kw or y
+                    new_data.name = label_name
+                else:
+                    match = is_list_like(label_kw) and len(label_kw) == len(y)
+                    if label_kw and not match:
+                        raise ValueError(
+                            "label should be list-like and same length as y"
+                        )
+                    label_name = label_kw or data[y].columns
+                    new_data.columns = label_name
 
                 for kw in ['xerr', 'yerr']:
                     if (kw in kwds) and \
@@ -1775,7 +1790,7 @@ def _plot(data, x=None, y=None, subplots=False,
                             kwds[kw] = data[kwds[kw]]
                         except (IndexError, KeyError, TypeError):
                             pass
-                data = series
+                data = new_data
         plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
 
     plot_obj.generate()
@@ -1788,7 +1803,7 @@ def _plot(data, x=None, y=None, subplots=False,
 series_kind = ""
 
 df_coord = """x : label or position, default None
-    y : label or position, default None
+    y : label, position or list of label, positions, default None
         Allows plotting of one column versus another"""
 series_coord = ""
 

pandas/tests/plotting/test_frame.py

@@ -2170,26 +2170,51 @@ def test_invalid_kind(self):
         with pytest.raises(ValueError):
             df.plot(kind='aasdf')
 
-    @pytest.mark.parametrize("x,y", [
-        (['B', 'C'], 'A'),
-        ('A', ['B', 'C'])
+    @pytest.mark.parametrize("x,y,lbl", [
+        (['B', 'C'], 'A', 'a'),
+        (['A'], ['B', 'C'], ['b', 'c']),
+        ('A', ['B', 'C'], 'badlabel')
     ])
-    def test_invalid_xy_args(self, x, y):
-        # GH 18671
+    def test_invalid_xy_args(self, x, y, lbl):
+        # GH 18671, 19699 allows y to be list-like but not x
         df = DataFrame({"A": [1, 2], 'B': [3, 4], 'C': [5, 6]})
         with pytest.raises(ValueError):
-            df.plot(x=x, y=y)
+            df.plot(x=x, y=y, label=lbl)
 
     @pytest.mark.parametrize("x,y", [
         ('A', 'B'),
-        ('B', 'A')
+        (['A'], 'B')
     ])
     def test_invalid_xy_args_dup_cols(self, x, y):
-        # GH 18671
+        # GH 18671, 19699 allows y to be list-like but not x
         df = DataFrame([[1, 3, 5], [2, 4, 6]], columns=list('AAB'))
         with pytest.raises(ValueError):
             df.plot(x=x, y=y)
 
+    @pytest.mark.parametrize("x,y,lbl,colors", [
+        ('A', ['B'], ['b'], ['red']),
+        ('A', ['B', 'C'], ['b', 'c'], ['red', 'blue']),
+        (0, [1, 2], ['bokeh', 'cython'], ['green', 'yellow'])
+    ])
+    def test_y_listlike(self, x, y, lbl, colors):
+        # GH 19699
+        df = DataFrame({"A": [1, 2], 'B': [3, 4], 'C': [5, 6]})
+        _check_plot_works(df.plot, x='A', y=y, label=lbl)
+
+        ax = df.plot(x=x, y=y, label=lbl, color=colors)
+        assert len(ax.lines) == len(y)
+        self._check_colors(ax.get_lines(), linecolors=colors)
+
+    @pytest.mark.parametrize("x,y,colnames", [
+        (0, 1, ['A', 'B']),
+        (1, 0, [0, 1])
+    ])
+    def test_xy_args_integer(self, x, y, colnames):
+        # GH 20056
+        df = DataFrame({"A": [1, 2], 'B': [3, 4]})
+        df.columns = colnames
+        _check_plot_works(df.plot, x=x, y=y)
+
     @pytest.mark.slow
     def test_hexbin_basic(self):
         df = self.hexbin_df