如何在matplotlib中繪制多個條形圖，當我嘗試多次調用bar函數(shù)時，它們重疊了，如下圖所示，只能看到最高的紅色。如何在X軸上繪制帶有日期的多個條形圖？到目前為止，我已經(jīng)嘗試過了：import matplotlib.pyplot as pltimport datetimex = [    datetime.datetime(2011, 1, 4, 0, 0),    datetime.datetime(2011, 1, 5, 0, 0),    datetime.datetime(2011, 1, 6, 0, 0)]y = [4, 9, 2]z = [1, 2, 3]k = [11, 12, 13]ax = plt.subplot(111)ax.bar(x, y, width=0.5, color='b', align='center')ax.bar(x, z, width=0.5, color='g', align='center')ax.bar(x, k, width=0.5, color='r', align='center')ax.xaxis_date()plt.show()