plt.plot issue in pandas with categorical index DataFrame

问题

I have a DataFrame with categorical index like so:

import pandas as pd
import matplotlib.pyplot as plt
%matplotlib notebook

accidents_by_day=pd.DataFrame({'num_accidents':[5659,5298,4917,4461,4181,4038,3985],
                           'weekday':[7,1,6,5,4,2,3]})

weekday_map={1:'Sunday',2:'Monday',3:'Tuesday',4:'Wednesday',5:'Thursday',6:'Friday',7:'Saturday'}
new_index=(pd.CategoricalIndex(accidents_by_day.weekday.map(weekday_map)).
       reorder_categories(new_categories=['Monday','Tuesday','Wednesday','Thursday',
                                          'Friday','Saturday','Sunday'],
                          ordered=True))
accidents_by_day.set_index(new_index,drop=True,inplace=True)
accidents_by_day.sort_index(inplace=True)

While The following works fine:

accidents_by_day.num_accidents.plot(kind='bar')

The plt.plot(accidents_by_day.num_accidents) gives an error

~\Anaconda3\lib\site-packages\matplotlib\axes\_base.py in _plot_args(self, tup, kwargs)
    390             func = self._makefill
    391 
--> 392         ncx, ncy = x.shape[1], y.shape[1]
    393         for j in xrange(max(ncx, ncy)):
    394             seg = func(x[:, j % ncx], y[:, j % ncy], kw, kwargs)

IndexError: tuple index out of range

and plt.plot([accidents_by_day.num_accidents]) produces an empty figure.

Could anyone explain what is happening here?

Thanks!

回答1:

plt.plot takes two arguments, x and y: plt.plot(x,y). If you only specify a single argument, plt.plot(y), it is assumed that you want to plot against the numbers 0, ..., len(y)-1. So what is possible here, is to plot

plt.plot(accidents_by_day.num_accidents.values)

The produced plot might however not be the desired one, since the dataframe index is not taken into account.

So you may stick to the usual plt.plot(x,y) and supply the index,

plt.plot(accidents_by_day.index.categories, accidents_by_day.num_accidents.values)

来源：https://stackoverflow.com/questions/48835096/plt-plot-issue-in-pandas-with-categorical-index-dataframe