Switch default colormap for scatterplot to 'Greys'

In #7780, I added colormap support to scatter, but choose to override the
default rainbow colormap (https://github.com/pydata/pandas/pull/7780#issuecomment-49533995).

I'm now regreting choosing 'RdBu' as the default colormap. I think a simple
black-white colormap is a more conservative and better default choice for
coloring scatter plots -- binary colormaps really only make sense if the data
is signed. 'Greys' is also the default colormap set by the seaborn package,
and @mwaskom has done far more thinking about colormaps than I have.

If possible, I would love to get this in before 0.15 is released, so we don't
have to worry about changing how anyone's plots look.

CC @sinhrks
Yes. http://wiki.scipy.org/Cookbook/Matplotlib/Show_colormaps

On Thu, Oct 16, 2014 at 5:49 AM, jreback <notifications@github.com> wrote:

>> @@ -1605,8 +1605,8 @@ def test_plot_scatter_with_c(self):
>>          axes = [df.plot(kind='scatter', x='x', y='y', c='z'),
>>                  df.plot(kind='scatter', x=0, y=1, c=2)]
>>          for ax in axes:
>> -            # default to RdBu
>> -            self.assertEqual(ax.collections[0].cmap.name, 'RdBu')
>> +            # default to Greys
>> +            self.assertEqual(ax.collections[0].cmap.name, 'Greys')
> this is actually 'Greys' and not 'greys' ?
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