假设我创建了一个DataFrame，如下所示：

import pandas as pd
s1 = pd.Series(['a', 'b', 'a', 'c', 'a', 'b'])
s2 = pd.Series(['a', 'f', 'a', 'd', 'a', 'f', 'f'])
d = pd.DataFrame({'s1': s1, 's2', s2})

真实的数据中的字符串有相当多的稀疏性。我想创建字符串出现的直方图，它看起来像是由d.hist（）生成的（例如：对于S1和S2（每个子图一个）具有子图）。
仅仅执行d.hist（）就会产生这样的错误：

/Library/Python/2.7/site-packages/pandas/tools/plotting.pyc in hist_frame(data, column, by, grid, xlabelsize, xrot, ylabelsize, yrot, ax, sharex, sharey, **kwds)
   1725         ax.xaxis.set_visible(True)
   1726         ax.yaxis.set_visible(True)
-> 1727         ax.hist(data[col].dropna().values, **kwds)
   1728         ax.set_title(col)
   1729         ax.grid(grid)

/Library/Python/2.7/site-packages/matplotlib/axes.pyc in hist(self, x, bins, range, normed, weights, cumulative, bottom, histtype, align, orientation, rwidth, log, color, label, stacked, **kwargs)
   8099             # this will automatically overwrite bins,
   8100             # so that each histogram uses the same bins
-> 8101             m, bins = np.histogram(x[i], bins, weights=w[i], **hist_kwargs)
   8102             if mlast is None:
   8103                 mlast = np.zeros(len(bins)-1, m.dtype)

/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/numpy/lib/function_base.pyc in histogram(a, bins, range, normed, weights, density)
    167             else:
    168                 range = (a.min(), a.max())
--> 169         mn, mx = [mi+0.0 for mi in range]
    170         if mn == mx:
    171             mn -= 0.5

TypeError: cannot concatenate 'str' and 'float' objects

我想我可以手动浏览每个系列，做一个value_counts()，然后将其绘制为条形图，并手动创建子图。我想看看有没有更简单的方法。