mdate formatter: intervals not displaying nicely

[KatiRG]

Dates on x-axis are not plotted in regular intervals on the EC2 Jupyterhub (python 3.5.2):

ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))

There was an issue raised about this:
matplotlib/matplotlib#13183
which was apparently solved here: matplotlib/matplotlib#10841

On Jupyterhub python version 3.7.3 the same code does produces equal intervals.