Timeline axis sometimes can have no minor lables

Consider following page

https://gist.github.com/jtraub/8d62b49554d14a8c5e1b

If you open it and try to change range using mouse wheel you will eventually see that sometimes minor labels on timeline axis can be missing.

This happens when display of ante/post meridiem is added using format option.

Debug shows that this behavior is caused by computations in TimeAxis._repaintLabels. It is assumed that minor labels won't exceed 7 characters width which is not true in case of, say, 07:30 AM.

Sorry, I can not provide patch for the bug right now. Going to take a deeper look and try to fix it in the next few days