A formatter for dates in matplotlib plots where the output format string changes based on a condition.

conditional_date_formatter.py

from matplotlib import ticker
from matplotlib.dates import DateFormatter, _from_ordinalf

class ConditionalDateFormatter(ticker.Formatter):
  def __init__(self, fmt: str, fmt_conditional: str, date_condition: Callable[[datetime], bool], tz=None):
    """
      A formatter for dates in matplotlib plots where the output format string changes based on a condition.

      Args:
        fmt (str): regular format string
        fmt_conditional (str): format string for when condition == True
        date_condition (Callable[[datetime], bool]): returns True when
            fmt_conditional is desired based on the given datetime.
        tz (): timezone for formatter.
    """
    self.formatter = DateFormatter(fmt, tz)
    self.conditional_formatter = DateFormatter(fmt_conditional, tz)
    self.date_condition = date_condition
    self.tz = tz

  def __call__(self, x, pos=0):
    dt = _from_ordinalf(x, tz=self.tz)
    if self.date_condition(dt):
      return self.conditional_formatter.__call__(x, pos)
    return self.formatter.__call__(x, pos)

example.py

regular_format = '%-I%p' 
conditional_format = '%b %-d'  
condition = lambda x: x.hour == 0  # aka midnight

formatter = ConditionalDateFormatter(regular_format, conditional_format, condition=condition)

# When used in a plot of datetimes the axis might look like:
# [..., 10PM, 11PM, Mar 13, 1AM, 2AM, 3AM, ...]