Bug report

Bug summary

In the example below date format is changed when plotting with plot_date after calling dates.DayLocator(). This can be fixed with using dates.DateFormatter(), but the confusion could be avoided if plot_date uses the same date format when it is not specified by user.

Code for reproduction

import datetime
import matplotlib.pyplot as plt
import matplotlib.dates as mdates

DATE1 = mdates.date2num(datetime.datetime(2017, 8, 10, 18, 0))
DATE2 = mdates.date2num(datetime.datetime(2017, 8, 11, 18, 0))
DATE3 = mdates.date2num(datetime.datetime(2017, 8, 12, 18, 0))
plt.plot_date([DATE1, DATE2, DATE3], [1, 2, 3], '-')
plt.show()

plt.plot_date([DATE1, DATE2, DATE3], [1, 2, 3], '-')
plt.gca().xaxis.set_major_locator(mdates.DayLocator())
plt.show()

Actual outcome

Matplotlib version

• Operating system: Mac OS
• Matplotlib version: 2.0.2
• Matplotlib backend (print(matplotlib.get_backend())):
• Python version: 2.7.13
• Jupyter version (if applicable):
• Other libraries: