Bug report

When plotting with plot_date(), the x axis ticks can be unevenly spaced. It looks like when the tick spacing is less than a month, the start of the month is always present as a tick, which may mean the space between it and the previous tick is small. This can lead to overlapping tick labels and generally poor looking axes and grids.

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt

start = np.datetime64('2018-01-01', 'D')
end = np.datetime64('2018-04-01', 'D')
dates = np.arange(start, end)
data = np.random.randn(len(dates))

plt.plot_date(dates, data)
plt.grid(True)
plt.gcf().autofmt_xdate()
plt.show()

Actual outcome

Expected outcome

The plot should have evenly spaced gridlines and axis ticks, unless a custom locator has specified otherwise.

Matplotlib version

• Operating system: Arch Linux
• Matplotlib version: 3.0.2
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.7.2
• Jupyter version (if applicable): n/a
• Other libraries: matplotlib 1.15.4

Matplotlib, Python and numpy are installed with the OS package manager using the standard Arch Linux repositories.