我有这个数据框：
date_obj col1 col2 col3 col4
40038 2012-11-19 1.000 0.831856 0.986209 0.843919
40039 2012-11-20 2.015 0.521764 1.177320 0.938245
40040 2012-11-21 1.160 1.645345 1.964620 4.536440
40041 2012-11-22 3.171 2.444018 2.931550 3.737840
40042 2012-11-23 4.563 3.208111 3.587250 2.434040
40043 2012-11-24 5.379 3.863732 3.824540 1.634780
40044 2012-11-26 1.125 20.756739 4.162820 23.552100
40045 2012-11-27 3.340 5.369354 4.535090 1.129290
40046 2012-11-28 5.463 12.185730 8.102790 1.224300
40047 2012-11-29 6.596 14.328685 9.271000 24.655600
40048 2012-11-30 31.544 13.513497 12.103400 21.273500
40049 2012-12-01 24.921 26.144050 16.256200 13.883100
40050 2012-12-03 5.488 2.581351 7.220790 3.349450
40051 2012-12-04 6.977 5.893819 5.548870 2.948770
40052 2012-12-05 7.115 6.533022 5.863820 2.517030
40053 2012-12-06 5.842 8.754232 7.518660 1.447940
40054 2012-12-07 6.346 12.018631 10.263100 11.837400
40055 2012-12-08 17.666 4.548846 10.610400 11.110800
40056 2012-12-10 4.300 2.823566 1.475000 1.989210
40057 2012-12-11 2.415 2.436319 2.677440 2.908270
40058 2012-12-12 2.319 2.121092 3.455550 3.890480
40059 2012-12-13 1.000 1.633918 3.858540 4.316940
40060 2012-12-14 2.238 1.688475 5.065990 5.267850
40061 2012-12-15 1.798 2.621267 7.175370 6.957340
我尝试按以下方式绘制它：
plt.figure(figsize=(17, 10))
plt.setp(plt.xticks()[1], rotation=45)
plt.plot_date(df_cut['date_obj'],df_cut['col1'], color='black', linestyle='-', markersize=3, linewidth=2)
plt.plot_date(df_cut['date_obj'],df_cut['col2'], color='red', linestyle='-', markersize=3)
plt.plot_date(df_cut['date_obj'],df_cut['col3'], color='green', linestyle='-', markersize=3)
plt.plot_date(df_cut['date_obj'],df_cut['col4'], color='blue', linestyle='-', markersize=3)
结果我得到一个错误：
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
<ipython-input-544-1b8650d1e7e7> in <module>()
/ipython/local/lib/python2.7/site-packages/matplotlib/pyplot.pyc in plot_date(x, y, fmt, tz, xdate, ydate, hold, **kwargs)
2850 try:
2851 ret = ax.plot_date(x, y, fmt=fmt, tz=tz, xdate=xdate, ydate=ydate,
-> 2852 **kwargs)
2853 draw_if_interactive()
2854 finally:
ipython/local/lib/python2.7/site-packages/matplotlib/axes.pyc in plot_date(self, x, y, fmt, tz, xdate, ydate, **kwargs)
4061 if not self._hold: self.cla()
4062
-> 4063 ret = self.plot(x, y, fmt, **kwargs)
4064
4065 if xdate:
ipython/local/lib/python2.7/site-packages/matplotlib/axes.pyc in plot(self, *args, **kwargs)
3994 lines = []
3995
-> 3996 for line in self._get_lines(*args, **kwargs):
3997 self.add_line(line)
3998 lines.append(line)
ipython/local/lib/python2.7/site-packages/matplotlib/axes.pyc in _grab_next_args(self, *args, **kwargs)
328 return
329 if len(remaining) <= 3:
--> 330 for seg in self._plot_args(remaining, kwargs):
331 yield seg
332 return
ipython/local/lib/python2.7/site-packages/matplotlib/axes.pyc in _plot_args(self, tup, kwargs)
306 x = np.arange(y.shape[0], dtype=float)
307
--> 308 x, y = self._xy_from_xy(x, y)
309
310 if self.command == 'plot':
python/local/lib/python2.7/site-packages/matplotlib/axes.pyc in _xy_from_xy(self, x, y)
222 def _xy_from_xy(self, x, y):
223 if self.axes.xaxis is not None and self.axes.yaxis is not None:
--> 224 bx = self.axes.xaxis.update_units(x)
225 by = self.axes.yaxis.update_units(y)
226
ipython/local/lib/python2.7/site-packages/matplotlib/axis.pyc in update_units(self, data)
1299 neednew = self.converter != converter
1300 self.converter = converter
-> 1301 default = self.converter.default_units(data, self)
1302 #print 'update units: default=%s, units=%s'%(default, self.units)
1303 if default is not None and self.units is None:
ipython/local/lib/python2.7/site-packages/matplotlib/dates.pyc in default_units(x, axis)
1156 'Return the tzinfo instance of *x* or of its first element, or None'
1157 try:
-> 1158 x = x[0]
1159 except (TypeError, IndexError):
1160 pass
ipython/local/lib/python2.7/site-packages/pandas/core/series.pyc in __getitem__(self, key)
611 def __getitem__(self, key):
612 try:
--> 613 return self.index.get_value(self, key)
614 except InvalidIndexError:
615 pass
ipython/local/lib/python2.7/site-packages/pandas/core/index.pyc in get_value(self, series, key)
761 """
762 try:
--> 763 return self._engine.get_value(series, key)
764 except KeyError, e1:
765 if len(self) > 0 and self.inferred_type == 'integer':
奇怪的是，此代码适用于某些数据帧，而对于某些数据帧则不起作用。数据帧的结构没有什么不同。它们之间的唯一区别仅在于它们包含的值。
有人可以帮我解决这个问题吗？