matplotlib plot_date() add vertical line at specified date

Question

My data is stored as a pandas dataframe. I have created a chart of date (object format) vs. percentile (int64 format) using the plot_date() function in matplotlib and would like to add some vertical lines at pre-specified dates.

I have managed to add point markers at the pre-specified dates, but can't seem to find a way to make them into vertical lines.

I've found various answers on SO/SE about how to add vertical lines to a plot(), but am having trouble converting my data to a format that can be used by plot(), hence why I have used plot_date().

Sample data:

date       percentile
2012-05-30  3
2014-11-25  60
2012-06-15  38
2013-07-18  16

My code to plot the chart is as below:

x_data = data["date"]
y_data = data["percentile"]

plt.figure()
plt.plot()

#create a scatter chart of dates vs. percentile
plt.plot_date(x = x_data, y = y_data)

#now add a marker at prespecified dates - ideally this would be a vertical line
plt.plot_date(x = '2012-09-21', y = 0)

plt.savefig("percentile_plot.png")
plt.close()

Unfortunately I can't provide an image of the current output as the code is on a terminal with no web access.

Any help is greatly appreciated - also in terms of how I've asked the question as I am quite new to SO / SE.

Thank you.

Answer 1

In MatPlotLib 1.4.3 this works:

import datetime as dt

plt.axvline(dt.datetime(2012, 9, 21))

Passing a string-style date (2012-09-21) doesn't work because MPL doesn't know this is a date. Whatever code you are using to load your file is probably implicitly creating datetime objects out of your strings, which is why the plot call works.

Also, in MPL 1.4.3, I did not need to call plt.plot_date(data['date'], ...), simply calling plt.plot(data['date'], ...) worked for me as long as the data['date'] column is a column of datetime objects.

Good luck.