Customize the styles of x-axis with dates.
import matplotlib.dates as mdates
plot.hist(mdates.date2num(dts), 100) # plot the histogram
ax = plt.gca()
ax.xaxis.set_major_locator(mdates.AutoDateLocator()) # set the major ticker, use AutoDateLocator
ax.xaxis.set_major_formatter(mdates.DateFormatter('%m-%d')) # show the label
ax.xaxis.set_minor_locator(mdates.DayLocator()) # set the minor ticker, use DayLocator