Plotting dates on the x-axis with Python's matplotlib


Question

I am trying to plot information against dates. I have a list of dates in the format "01/02/1991".

I converted them by doing the following:

x = parser.parse(date).strftime('%Y%m%d'))

which gives 19910102

Then I tried to use num2date

import matplotlib.dates as dates
new_x = dates.num2date(x)

Plotting:

plt.plot_date(new_x, other_data, fmt="bo", tz=None, xdate=True)

But I get an error. It says "ValueError: year is out of range". Any solutions?

1
53
3/9/2012 1:47:57 AM

Accepted Answer

As @KyssTao has been saying, help(dates.num2date) says that the x has to be a float giving the number of days since 0001-01-01 plus one. Hence, 19910102 is not 2/Jan/1991, because if you counted 19910101 days from 0001-01-01 you'd get something in the year 54513 or similar (divide by 365.25, number of days in a year).

Use datestr2num instead (see help(dates.datestr2num)):

new_x = dates.datestr2num(date) # where date is '01/02/1991'
18
3/9/2012 1:58:22 AM

You can do this more simply using plot() instead of plot_date().

First, convert your strings to instances of Python datetime.date:

import datetime as dt

dates = ['01/02/1991','01/03/1991','01/04/1991']
x = [dt.datetime.strptime(d,'%m/%d/%Y').date() for d in dates]
y = range(len(x)) # many thanks to Kyss Tao for setting me straight here

Then plot:

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

plt.gca().xaxis.set_major_formatter(mdates.DateFormatter('%m/%d/%Y'))
plt.gca().xaxis.set_major_locator(mdates.DayLocator())
plt.plot(x,y)
plt.gcf().autofmt_xdate()

Result:

enter image description here


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