Date locator for DAILY interval now returns middle of month
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The `matplotlib.dates.AutoDateLocator` has a default of
``interval_multiples=True`` that attempts to align ticks with the start of
meaningful intervals like the start of the month, or start of the day, etc.
That lead to approximately 140-day intervals being mapped to the first and 22nd
of the month.  This has now been changed so that it chooses the first and
15th of the month, which is probably what most people want.