Fix matplotlib warnings
This is with matplotlib 2.1.0

```
===================================================================== warnings summary ======================================================================
pandas/tests/plotting/test_frame.py::TestDataFramePlots::()::test_subplots_multiple_axes
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_tools.py:204: UserWarning: When passing multiple axes, layout keyword is ignored
    "ignored", UserWarning)

pandas/tests/plotting/test_frame.py::TestDataFramePlots::()::test_line_colors
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_core.py:188: UserWarning: 'colors' is being deprecated. Please use 'color'instead of 'colors'
    warnings.warn(("'colors' is being deprecated. Please use 'color'"

pandas/tests/plotting/test_frame.py::TestDataFramePlots::()::test_errorbar_plot
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_core.py:1727: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    plot_obj.generate()
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_core.py:1727: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    plot_obj.generate()
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_core.py:1727: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    plot_obj.generate()

pandas/tests/plotting/test_frame.py::TestDataFramePlots::()::test_errorbar_timeseries
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_core.py:1727: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    plot_obj.generate()
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_core.py:1727: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    plot_obj.generate()
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/plotting/_core.py:1727: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    plot_obj.generate()

pandas/tests/plotting/test_hist_method.py::TestSeriesPlots::()::test_hist_legacy
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)

pandas/tests/plotting/test_hist_method.py::TestDataFramePlots::()::test_tight_layout
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/tests/plotting/test_hist_method.py:248: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    _check_plot_works(df.hist)

pandas/tests/plotting/test_misc.py::TestSeriesPlots::()::test_autocorrelation_plot
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)

pandas/tests/plotting/test_misc.py::TestDataFramePlots::()::test_andrews_curves
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)

pandas/tests/plotting/test_misc.py::TestDataFramePlots::()::test_parallel_coordinates_with_sorted_labels
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/axes/_base.py:2918: UserWarning: Attempting to set identical left==right results
  in singular transformations; automatically expanding.
  left=0, right=0
    'left=%s, right=%s') % (left, right))

pandas/tests/plotting/test_misc.py::TestDataFramePlots::()::test_radviz
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/pyplot.py:959: UserWarning: Requested projection is different from current axis projection, creating new axis with requested projection.
    return gcf().gca(**kwargs)

pandas/tests/plotting/test_series.py::TestSeriesPlots::()::test_hist_legacy
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
  /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance.  In a future version, a new instance will always be created and returned.  Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
    warnings.warn(message, mplDeprecation, stacklevel=1)
```