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.

farenorth · Accepted Answer · 2016-01-12 22:20:41Z

36

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.

answered Jan 12, 2016 at 22:20

farenorth

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2 Comments

annievic Over a year ago

Thank you for your fast response. I successfully used pandas.to_datetime(data ['date']) to convert the date field to datetime62 [ns]. However, when I use plt.plot(x = data ['date'], y = data['percentile']) then the resulting chart is completely blank with both axis ranging from -0.06 to +0.06.

Loek Over a year ago

Still works fine in matplotlib 3.0.2., even with subplots that don't return their axes.

ntg · Accepted Answer · 2023-07-03 22:34:56Z

21

Use pandas. Here is an example, also converting to timestamps just in case:

 df = pd.DataFrame({
    'date':[
        '2012-05-30',
        '2014-11-25',
        '2012-06-15',
        '2013-07-18',
    ],
    'percentile':[3,60,38,16]
})
df['date'] = df['date'].apply(pd.Timestamp)
df = df.set_index('date')
plt = df.plot(marker='o')
plt.axvline(pd.Timestamp('2013-09-21'),color='r')

Here is the output:

edited Jul 3, 2023 at 22:34

answered Oct 5, 2018 at 12:48

ntg

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1 Comment

Gombo Over a year ago

I think this line df.plot(marker='o') should be plt=df.plot(marker='o')

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