python - Annotate Time Series plot in Matplotlib

Question

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

Answer 1

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()