scatter() does not work with x/y variables and metadata kwargs

[gidden]

It seems like there is an issue if you want to use color/marker based on metadata

(df
 .filter(region='World')
 .filter(Temperature='uncategorized', keep=False)
 .scatter(x='Primary Energy|Coal', y='Emissions|CO2', color='Temperature', 
          legend=True)
)

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/.local/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba(c, alpha)
    157     try:
--> 158         rgba = _colors_full_map.cache[c, alpha]
    159     except (KeyError, TypeError):  # Not in cache, or unhashable.

KeyError: ('T', None)

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
~/.local/lib/python3.5/site-packages/matplotlib/axes/_axes.py in scatter(self, x, y, s, c, marker, cmap, norm, vmin, vmax, alpha, linewidths, verts, edgecolors, **kwargs)
   4209             try:  # Then is 'c' acceptable as PathCollection facecolors?
-> 4210                 colors = mcolors.to_rgba_array(c)
   4211                 n_elem = colors.shape[0]

~/.local/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba_array(c, alpha)
    258     for i, cc in enumerate(c):
--> 259         result[i] = to_rgba(cc, alpha)
    260     return result

~/.local/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba(c, alpha)
    159     except (KeyError, TypeError):  # Not in cache, or unhashable.
--> 160         rgba = _to_rgba_no_colorcycle(c, alpha)
    161         try:

~/.local/lib/python3.5/site-packages/matplotlib/colors.py in _to_rgba_no_colorcycle(c, alpha)
    203             pass
--> 204         raise ValueError("Invalid RGBA argument: {!r}".format(orig_c))
    205     # tuple color.

ValueError: Invalid RGBA argument: 'T'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-35-12664c2e4825> in <module>()
      3  .filter(Temperature='uncategorized', keep=False)
      4  .scatter(x='Primary Energy|Coal', y='Emissions|CO2', color='Temperature', 
----> 5           legend=True)
      6 )

~/.local/lib/python3.5/site-packages/pyam/core.py in scatter(self, x, y, **kwargs)
    958                 .rename(columns={'value': var})
    959             )
--> 960         ax = plotting.scatter(df, x, y, **kwargs)
    961         return ax
    962 

~/.local/lib/python3.5/site-packages/pyam/plotting.py in scatter(df, x, y, ax, legend, title, color, marker, linestyle, cmap, groupby, with_lines, **kwargs)
    603         else:
    604             kwargs.pop('linestyle')  # scatter() can't take a linestyle
--> 605             ax.scatter(group[x], group[y], **kwargs)
    606 
    607     # build legend handles and labels

~/.local/lib/python3.5/site-packages/matplotlib/__init__.py in inner(ax, data, *args, **kwargs)
   1783                         "the Matplotlib list!)" % (label_namer, func.__name__),
   1784                         RuntimeWarning, stacklevel=2)
-> 1785             return func(ax, *args, **kwargs)
   1786 
   1787         inner.__doc__ = _add_data_doc(inner.__doc__,

~/.local/lib/python3.5/site-packages/matplotlib/axes/_axes.py in scatter(self, x, y, s, c, marker, cmap, norm, vmin, vmax, alpha, linewidths, verts, edgecolors, **kwargs)
   4229                     "or as numbers to be mapped to colors. "
   4230                     "Here c = {}."  # <- beware, could be long depending on c.
-> 4231                     .format(c)
   4232                 )
   4233         else:

ValueError: 'c' argument must either be valid as mpl color(s) or as numbers to be mapped to colors. Here c = Temperature.