New issue 745: Can't pl.plot(YTArray, YTArray)
https://bitbucket.org/yt_analysis/yt/issue/745/cant-plplot-ytarray-ytarray
Sam Skillman:
```
#!python
from yt.mods import *
import matplotlib.pyplot as plt
a = YTArray([1.0,2.0])
plt.plot(a, a)
```
produces:
```
#!python
(yt-x86_64)360Mpc/1024L0:python test_plot.py
Traceback (most recent call last): File "test_plot.py", line 6, in <module>
plt.plot(a, a)
File "/u/ki/skillman/yt-x86_64/lib/python2.7/site-packages/matplotlib-1.3.0-py2.7-linux-x86_64.egg/matplotlib/pyplot.py", line 2987, in plot
ret = ax.plot(*args, **kwargs)
File "/u/ki/skillman/yt-x86_64/lib/python2.7/site-packages/matplotlib-1.3.0-py2.7-linux-x86_64.egg/matplotlib/axes.py", line 4137, in plot
for line in self._get_lines(*args, **kwargs):
File "/u/ki/skillman/yt-x86_64/lib/python2.7/site-packages/matplotlib-1.3.0-py2.7-linux-x86_64.egg/matplotlib/axes.py", line 317, in _grab_next_args
for seg in self._plot_args(remaining, kwargs):
File "/u/ki/skillman/yt-x86_64/lib/python2.7/site-packages/matplotlib-1.3.0-py2.7-linux-x86_64.egg/matplotlib/axes.py", line 276, in _plot_args
linestyle, marker, color = _process_plot_format(tup[-1])
File "/u/ki/skillman/yt-x86_64/lib/python2.7/site-packages/matplotlib-1.3.0-py2.7-linux-x86_64.egg/matplotlib/axes.py", line 80, in _process_plot_format
color = mcolors.colorConverter.to_rgb(fmt)
File "/u/ki/skillman/yt-x86_64/lib/python2.7/site-packages/matplotlib-1.3.0-py2.7-linux-x86_64.egg/matplotlib/colors.py", line 291, in to_rgb
argl = arg.lower()
AttributeError: 'YTArray' object has no attribute 'lower'
```
Using .to_ndarray() on a in the plotting call works as expected. pl.plot(a) also works, so I think this must be something about mpl interpreting the second ytarray as a different arg/kwarg.
Responsible: ngoldbaum