Issue 1:
The following code raises an error (tested on pandas 0.17.0, matplotlib 1.4.3, and Python 3.4.2)

>>> fig, ax = plt.subplots()
>>> fig.add_axes([0.2, 0.2, 0.2, 0.2])
>>> pd.Series(np.random.rand(100)).plot(ax=ax)
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-13-80531efe48c4> in <module>()
8 fig, ax = plt.subplots()
9 inset = fig.add_axes([0.2, 0.2, 0.2, 0.2])
---> 10 data.plot(ax=ax)

/Users/jakevdp/anaconda/envs/python3.4/lib/python3.4/site-packages/pandas/tools/plotting.py in __call__(self, kind, ax, figsize, use_index, title, grid, legend, style, logx, logy, loglog, xticks, yticks, xlim, ylim, rot, fontsize, colormap, table, yerr, xerr, label, secondary_y, **kwds)
3491 colormap=colormap, table=table, yerr=yerr,
3492 xerr=xerr, label=label, secondary_y=secondary_y,
-> 3493 **kwds)
3494 __call__.__doc__ = plot_series.__doc__
3495

/Users/jakevdp/anaconda/envs/python3.4/lib/python3.4/site-packages/pandas/tools/plotting.py in plot_series(data, kind, ax, figsize, use_index, title, grid, legend, style, logx, logy, loglog, xticks, yticks, xlim, ylim, rot, fontsize, colormap, table, yerr, xerr, label, secondary_y, **kwds)
2581 yerr=yerr, xerr=xerr,
2582 label=label, secondary_y=secondary_y,
-> 2583 **kwds)
2584
2585

/Users/jakevdp/anaconda/envs/python3.4/lib/python3.4/site-packages/pandas/tools/plotting.py in _plot(data, x, y, subplots, ax, kind, **kwds)
2378 plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
2379
-> 2380 plot_obj.generate()
2381 plot_obj.draw()
2382 return plot_obj.result

/Users/jakevdp/anaconda/envs/python3.4/lib/python3.4/site-packages/pandas/tools/plotting.py in generate(self)
990 self._post_plot_logic_common(ax, self.data)
991 self._post_plot_logic(ax, self.data)
--> 992 self._adorn_subplots()
993
994 def _args_adjust(self):

/Users/jakevdp/anaconda/envs/python3.4/lib/python3.4/site-packages/pandas/tools/plotting.py in _adorn_subplots(self)
1141 naxes=nrows * ncols, nrows=nrows,
1142 ncols=ncols, sharex=self.sharex,
-> 1143 sharey=self.sharey)
1144
1145 for ax in self.axes:

/Users/jakevdp/anaconda/envs/python3.4/lib/python3.4/site-packages/pandas/tools/plotting.py in _handle_shared_axes(axarr, nplots, naxes, nrows, ncols, sharex, sharey)
3378 layout = np.zeros((nrows+1,ncols+1), dtype=np.bool)
3379 for ax in axarr:
-> 3380 layout[ax.rowNum, ax.colNum] = ax.get_visible()
3381
3382 for ax in axarr:

AttributeError: 'Axes' object has no attribute 'rowNum'

Issue 2:
When attempting to use the Pandas 0.17.0 plot method to target plotting on an external axes: AttributeError: 'Axes' object has no attribute 'is_first_col'.

Pandas: 0.17.0

Numpy: 1.9.2

Matplotlib: 1.5.0

import pandas as pd
import numpy as np
from matplotlib import pyplot as plt

df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
'foo', 'bar', 'foo', 'foo'],
'B' : ['one', 'one', 'two', 'three',
'two', 'two', 'one', 'three'],
'C' : np.random.randn(8),
'D' : np.random.randn(8)})

fig = plt.figure()
ax = fig.add_axes((0,0,1,1))

df.plot(kind='scatter', ax=ax, x='C', y='D', c=df.C, s=100, linewidth=0, colormap='hsv')
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-21-b38f872a3fc3> in <module>()
----> 1 df.plot(kind='scatter', ax=ax, x='C', y='D', c=df.C, s=100, linewidth=0)

D:\Python27\lib\site-packages\pandas\tools\plotting.pyc in __call__(self, x, y, kind, ax, subplots, sharex, sharey, layout, figsize, use_index, title, grid, legend, style, logx, logy, loglog, xticks, yticks, xlim, ylim, rot, fontsize, colormap, table, yerr, xerr, secondary_y, sort_columns, **kwds)
3665 fontsize=fontsize, colormap=colormap, table=table,
3666 yerr=yerr, xerr=xerr, secondary_y=secondary_y,
-> 3667 sort_columns=sort_columns, **kwds)
3668 __call__.__doc__ = plot_frame.__doc__
3669

D:\Python27\lib\site-packages\pandas\tools\plotting.pyc in plot_frame(data, x, y, kind, ax, subplots, sharex, sharey, layout, figsize, use_index, title, grid, legend, style, logx, logy, loglog, xticks, yticks, xlim, ylim, rot, fontsize, colormap, table, yerr, xerr, secondary_y, sort_columns, **kwds)
2550 yerr=yerr, xerr=xerr,
2551 secondary_y=secondary_y, sort_columns=sort_columns,
-> 2552 **kwds)
2553
2554

D:\Python27\lib\site-packages\pandas\tools\plotting.pyc in _plot(data, x, y, subplots, ax, kind, **kwds)
2378 plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
2379
-> 2380 plot_obj.generate()
2381 plot_obj.draw()
2382 return plot_obj.result

D:\Python27\lib\site-packages\pandas\tools\plotting.pyc in generate(self)
990 self._post_plot_logic_common(ax, self.data)
991 self._post_plot_logic(ax, self.data)
--> 992 self._adorn_subplots()
993
994 def _args_adjust(self):

D:\Python27\lib\site-packages\pandas\tools\plotting.pyc in _adorn_subplots(self)
1141 naxes=nrows * ncols, nrows=nrows,
1142 ncols=ncols, sharex=self.sharex,
-> 1143 sharey=self.sharey)
1144
1145 for ax in self.axes:

D:\Python27\lib\site-packages\pandas\tools\plotting.pyc in _handle_shared_axes(axarr, nplots, naxes, nrows, ncols, sharex, sharey)
3403 # as we only have labels in teh first column and we always have a subplot there,
3404 # we can skip the layout test
-> 3405 if ax.is_first_col():
3406 continue
3407 if sharey or len(ax.get_shared_y_axes().get_siblings(ax)) > 1:

AttributeError: 'Axes' object has no attribute 'is_first_col'