showing milliseconds in matplotlib

Question

I'm using matplotlib to plot data as a function of time in hh:mm:ss.ms format where ms is milliseconds. However, I don't see the milliseconds in the plot. Is it possible to add them as well?

dates = matplotlib.dates.datestr2num(x_values) # convert string dates to numbers
plt.plot_date(dates, y_values)  # doesn't show milliseconds

Answer 1

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The problem here is that there is a class to format ticks, and plot_date sets that class to something that you don't want: an automatic formatter that never plots milliseconds.

In order to change this, you need to change from matplotlib.dates.AutoDateFormatter to your own formatter. matplotlib.dates.DateFormatter(fmt) creates a formatter with a datetime.strftime format string. I'm not sure how to get this to show milliseconds, but it will show microseconds, which I hope will work for you; it's just one extra zero, after all. Try this code:

dates = matplotlib.dates.datestr2num(x_values) # convert string dates to numbers
plt.plot_date(dates, y_values)  # doesn't show milliseconds by default.

# This changes the formatter.
plt.gca().xaxis.set_major_formatter(matplotlib.dates.DateFormatter("%H:%M:%S.%f"))

# Redraw the plot.
plt.draw()