doc/source/visualization.rst links to missing image files
Here is the doc build log:

```
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/series_plot_basic2.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/frame_plot_basic_noleg.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/frame_plot_subplots.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/series_plot_logy.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/df_plot_xy.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/series_plot_secondary_y.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/frame_plot_secondary_y.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/frame_plot_secondary_y_no_right.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/ser_plot_suppress.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/ser_plot_suppress_parm.png
/tmp/pandas/doc/source/visualization.rst:None: WARNING: image file not readable: _static/ser_plot_suppress_context.png
```
