Time histogram bar label cleanup

- bar labels no longer ever different font sizes
- labels lined up on common baseline
- labels for weekdays and months now human-readable (M T W; Jan Feb)
- Saturday and Sunday highlighted in weekday labeling
- for other labels the beginning of a new cycle (new minute for seconds etc)
  highlighted
- more assurances that bar labels will be correct as possible
  - labels turned off if bar interval has a remainder (not quite an hour etc)
  - intervals with remainders now marked with `~` on axis label to indicate
    inexactness