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I have an index array (x) of dates (datetime objects) and an array of actual values (y: bond prices). Doing (in iPython):

plot(x,y)

Produces a perfectly fine time series graph with the x axis labeled with the dates. No problem so far. But I want to add text on certain dates. For example, at 2009-10-31 I wish to display the text "Event 1" with an arrow pointing to the y value at that date.

I have read trough the Matplotlib documentation on text() and annotate() to no avail. It only covers standard numbered x-axises, and I can´t infer how to work those examples on my problem.

Thank you

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up vote 31 down vote accepted

Matplotlib uses an internal floating point format for dates.

You just need to convert your date to that format (using matplotlib.dates.date2num or matplotlib.dates.datestr2num) and then use annotate as usual.

As a somewhat excessively fancy example:

import datetime as dt
import matplotlib.pyplot as plt
import matplotlib.dates as mdates

x = [dt.datetime(2009, 05, 01), dt.datetime(2010, 06, 01), 
     dt.datetime(2011, 04, 01), dt.datetime(2012, 06, 01)]
y = [1, 3, 2, 5]

fig, ax = plt.subplots()
ax.plot_date(x, y, linestyle='--')

ax.annotate('Test', (mdates.date2num(x[1]), y[1]), xytext=(15, 15), 
            textcoords='offset points', arrowprops=dict(arrowstyle='-|>'))

fig.autofmt_xdate()
plt.show()

enter image description here

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Dear Joe, wonderful. Thank you! – luffe Jun 17 '12 at 8:54

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