Using axvspan for date ranges in matplotlib

Thanks for the help ImportanceOfBeingErnest, for any one reading this is how I made a plot with winter/summer backgrounds and single labels:

yearlist = ['2013','2014','2015','2016','2017','2018','2019']

fig, ax = plt.subplots(figsize=(20, 10))
for i in range(len(yearlist)):
    if yearlist[i] == '2013':
        ax.axvspan(date2num(datetime.datetime(2012,10,15)), date2num(datetime.datetime(int(yearlist[i]),5,15)), 
           label="winter (15 october - 15 may)", color="crimson", alpha=0.3)
        ax.axvspan(date2num(datetime.datetime(int(yearlist[i]),5,15)), date2num(datetime.datetime(int(yearlist[i]),10,15)), 
           label="summer (15 may - 15 october)", color="blue", alpha=0.3)
    else:
        ax.axvspan(date2num(datetime.datetime(int(yearlist[i-1]),10,15)), date2num(datetime.datetime(int(yearlist[i]),5,15))
           , color="crimson", alpha=0.3)
        ax.axvspan(date2num(datetime.datetime(int(yearlist[i]),5,15)), date2num(datetime.datetime(int(yearlist[i]),10,15)), 
            color="blue", alpha=0.3)

In matplotlib datetime axes also use numbers, namely

Matplotlib represents dates using floating point numbers specifying the number of days since 0001-01-01 UTC, plus 1.

Many functions like plot, scatter, bar etc. automatically convert datetime objects to those numbers, whereas many helper functions, like axvspan, did not do this automatic conversion until recent versions of matplotlib.

So in matplotlib 3 you can easily do

ax.axvspan(datetime(2019,3,1), datetime(2019,3,31))

but if using an older versionyou need to do it manually, using matplotlib.dates.date2num, e.g.

ax.axvspan(date2num(datetime(2019,3,1)), date2num(datetime(2019,3,31)))

Some complete example:

from datetime import datetime
import matplotlib.pyplot as plt
from matplotlib.dates import date2num

fig, ax = plt.subplots()

ax.plot([datetime(2019,2,14), datetime(2019,4,26)], [1,2])

ax.axvspan(date2num(datetime(2019,3,1)), date2num(datetime(2019,3,31)), 
           label="March", color="crimson", alpha=0.3)

ax.legend()
fig.autofmt_xdate()
plt.show()