Hello, per api doc examples:
http://matplotlib.org/api/dates_api.html

users are lead to believe that they can use just plainly use "MO" such as:
loc = WeekdayLocator(byweekday=MO)

But when I execute the code above, I get a 'MO' not defined error.

So I looked at the weekdaylocator api doc:
http://matplotlib.org/api/dates_api.html#matplotlib.dates.WeekdayLocator

It says the following about the byweekday parameter:
"Mark every weekday in byweekday; byweekday can be a number or sequence.

Elements of byweekday must be one of MO, TU, WE, TH, FR, SA, SU, the constants from dateutils.rrule."

After reading the above, I modified my code to be the following after importing dateutil:
loc = WeekdayLocator(byweekday=dateutil.rrule.MO)
or loc = WeekdayLocator(byweekday=0)

Now, my script executes ok. By the way, the doc says "dateutils.rrule", it should leave out the "s" and instead be "dateutil.rrule", so that is another fix that needs to be made in the documentation.