Plotting with times - axis labels and eliding times

I'm using matplotlib.pyplot.plot to plot values from CSV files where
the x axis data are datetime objects. This works well except for two

    * The time ranges are large (one-second samples over a period of
      several days) and I find the x axis label formats are often
      inappropriate to the scale. How to I control the format of
      the x axis labels?

    * Samples are only collected for part of the time each day, say
      from 01:00 to 15:00. How can I elide the gaps and perhaps
      plot a vertical line to signify the elided gaps?


Skip Montanaro