I'm confused by this
setp(xax, 'major_locator', HourLocator(byhour=(0,12)))
# Hide every other tick; for some reason this means every _fourth_ tickline
why would you ask for ticks midnight and noon, and then hide half of
them. Why not just ask for them at noon
xax.set_major_locator(HourLocator(byhour=(12,)))
JDH