More helpful error message when DataFrame.plot fails on some column
I'm trying to plot a DataFrame and it wasn't working, so I tried loading the data set from the read_csv docs. It also failed. I think I'm missing something, but I've been successful with matplotlib before. :)

```
from pandas import read_csv
import matplotlib.pyplot as plt

junk = read_csv('bar.csv')
fig = plt.figure()
junk.plot()
fig.savefig('/tmp/foo3.png')
```

bar.csv

```
year,indiv,zit,xit
1977,"A",1.2,.6
1977,"B",1.5,.5
1977,"C",1.7,.8
1978,"A",.2,.06
1978,"B",.7,.2
1978,"C",.8,.3
1978,"D",.9,.5
1978,"E",1.4,.9
1979,"C",.2,.15
1979,"D",.14,.05
1979,"E",.5,.15
1979,"F",1.2,.5
1979,"G",3.4,1.9
1979,"H",5.4,2.7
1979,"I",6.4,1.2
```

I get the folowing error:

```
python /tmp/plot.py 
Traceback (most recent call last):
  File "/tmp/plot.py", line 6, in <module>
    junk.plot()
  File "/home/mharrison/work/pandas/env/lib/python2.6/site-packages/pandas/core/frame.py", line 3225, in plot
    ax.plot(x, y, label=str(col), **kwds)
  File "/home/mharrison/work/pandas/env/lib/python2.6/site-packages/matplotlib/axes.py", line 3783, in plot
    for line in self._get_lines(*args, **kwargs):
  File "/home/mharrison/work/pandas/env/lib/python2.6/site-packages/matplotlib/axes.py", line 317, in _grab_next_args
    for seg in self._plot_args(remaining, kwargs):
  File "/home/mharrison/work/pandas/env/lib/python2.6/site-packages/matplotlib/axes.py", line 275, in _plot_args
    linestyle, marker, color = _process_plot_format(tup[-1])
  File "/home/mharrison/work/pandas/env/lib/python2.6/site-packages/matplotlib/axes.py", line 93, in _process_plot_format
    if fmt.find('--')>=0:
AttributeError: 'numpy.ndarray' object has no attribute 'find'
```
