resolve issue 368
two commits to resolve [issue#368](https://github.com/wesm/pandas/issues/368)

thanks Wouter, I made sure that the other plot methods return something too-- will also write some unit tests to hit all of the cases described in the issue before releasing.
 Been thinking a bit on uniformering signatures for plot, hist, boxplot and what they do/return.
For reference, below an overview what pandas and matplotlib.pyplot have.

some things that come to mind
- +1 for what you did with boxplot, _by_ and _column_ are handy, this could also be used for other kind of plots.
- bar-plot goes through plot, why not bar() itself.
- maybe pie() to?
- plot offers possibility to control _sharex_ and _sharey_, others don not have this control.
- not all functions use the same subplot layout approach. DataFrame.plot() can plot all lines on a single axis or a nx1 layout. DataFrame.hist() uses nxn layout and DataFrame.boxplot is clever and can do nxm, but the user has no control. Maybe add _nrow_, _ncol_ arguments? Default to None, meaning pandas can control layout, if either one defined pandas should compute the other one. Can get tricky for plot(), need to do something with _subplots_ argument
- usage of **_kwds_, e.g boxplot has it, does not use this. maybe add _subplot_kw_ and _figure_kw_ arguments for dispatching argmuments -- like matplotlib does.
- Series.plot has _style_, DataFrame.plot not -- later could maybe use style/colum (like _formatters_ in to_string)?
- rot not used everywhere
- probably many users of pandas are familiair with matplotlib too, in general allign plotting signatures and return objects with matplotlib would be a good thing to do?

Maybe if i stare at it a bit longer i may have some more ideas, but this is getting long already. What do you think?

# for reference

## Series:

```
plot(self, label=None, kind='line', use_index=True, rot=30, ax=None, style='-',
     grid=True, **kwds)

hist(self, ax=None, grid=True, **kwds)
```

## DataFrame:

```
boxplot(self, column=None, by=None, ax=None, fontsize=None,
            rot=0, grid=True, **kwds)

plot(self, subplots=False, sharex=True, sharey=False, use_index=True,
         figsize=None, grid=True, legend=True, rot=30, ax=None,
         kind='line', **kwds)

def hist(self, grid=True, **kwds):
```

## matplotlib.pyplot

```
figure(num=None, figsize=None, dpi=None, facecolor=None, edgecolor=None,
             frameon=True, FigureClass=<class 'matplotlib.figure.Figure'>,
             **kwargs)

fig, ax = subplots(nrows=1, ncols=1, sharex=False, sharey=False, squeeze=True,
                   subplot_kw=None, **fig_kw)
---> always creates a new figure

plot(*args, **kwargs)
    returns list of matplotlib.lines.Line2D

boxplot(x, notch=0, sym='b+', vert=1, whis=1.5, positions=None,
            widths=None, patch_artist=False, bootstrap=None, hold=None)
    Returns a dictionary, mapping each component of the boxplot
    to a list of the :class:`matplotlib.lines.Line2D`
    instances created.

plt.pie(x, explode=None, labels=None, colors=None, autopct=None,
        pctdistance=0.6, shadow=False, labeldistance=1.1, hold=None)
   Return value:
      If *autopct* is None, return the tuple (*patches*, *texts*):

        - *patches* is a sequence of
          :class:`matplotlib.patches.Wedge` instances

        - *texts* is a list of the label
          :class:`matplotlib.text.Text` instances.

      If *autopct* is *None*, return the tuple (*patches*, *texts*)
      If *autopct* is not *None*, return the tuple (*patches*, *texts*, *autotexts*)

plt.bar(left, height, width=0.8, bottom=None, hold=None, **kwargs)
    Return value is a list of matplotlib.patches.Rectangle instances
```
 hi wouter, i'll move your comment to a new issue
