How to set dates in x axis with non-uniform interval?

Question

I would like to draw a time-plot using the matplotlib.dates module. From the tutorial, I know how to set regular interval date like the following:

ax.xaxis.set_major_locator(DayLocator())
ax.xaxis.set_major_formatter(DateFormatter("%Y-%m"))
ax.xaxis.set_minor_...

However, in my data there are some months missing. My xaxis date is more like : Jan, Feb, May, Jul, Sep, Oct, Sep... no pattern is formed...

How can I deal with such situation?.

Do you want to create a scatter plot, x-axis having month and y axis being values? — yosukesabai
– yosukesabai, Commented Nov 2, 2011 at 20:29
how are you data for x axis then? cant you just like create array for time, array for value, and feed like plot(times, values), like that? sorry if not i am not getting your point... — yosukesabai
– yosukesabai, Commented Nov 2, 2011 at 23:51
Yes, it is also a good solution...But how can I set the x axix into date format? I mean I want the x axis appears like 2009-08, 2010-10 this kind of thing. Thanks in advance. — AnneS
– AnneS, Commented Nov 3, 2011 at 17:06
I use ticker to manually set the xaxis.setMajorFormatter(FixedFormatter(dateStr)) into the date format I want. And I use xaxis.set_major_locator(IndexLocator(1, 0)) to set the location of the date str... However, do you have a more decent way to do the same thing? — AnneS
– AnneS, Commented Nov 3, 2011 at 17:37

Mauro · Accepted Answer · 2011-11-04 22:11:58Z

2

If I understand correctly you may need to use

matplotlib.ticker.FixedLocator

Example which puts labels at data points 1, 10 and the last one:

import pylab as plt
from matplotlib.ticker import FixedLocator
from matplotlib.dates import DateFormatter
import numpy as np
from datetime import datetime

# dates (unequally spaced, but works exactly same for equally spaced dates too)
dates = plt.date2num(datetime(2010,3,1))  + np.array([0, 10, 50, 100, 180, 300])
# just some data
data = dates**2
plt.plot_date(dates,data)

ax = plt.gca()
ax.xaxis.set_major_locator(FixedLocator(dates[[1, 3, len(dates)-1]]))
# for some reason this is needed to get the month displayed too
ax.xaxis.set_major_formatter(DateFormatter("%Y-%m"))
plt.show()

edited Nov 4, 2011 at 22:11

answered Nov 4, 2011 at 7:09

Mauro

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3 Comments

yosukesabai Over a year ago

This does change label format. The only thing I needed was from matplotlib.dates import DateFormatter

Mauro Over a year ago

Hmm, AnneS's question is a bit unclear. But just using plot_date will put equally spaced ticks and labels on the plot even for unequally spaced data. So, I figured that the question was 'How to set dates labels in x axis with ununiform interval', which above answers.

Mauro Over a year ago

@yosukesabai Well, as stated in above comment, I don't think DateFormatter is actually needed so I removed it from the code.

yosukesabai · Accepted Answer · 2011-11-04 12:11:32Z

Starting from Mauro's solution, i tried to plot data pair where dates are irregularly spaced, so that i can find this QA sometime in future.

import pylab as plt
from matplotlib.ticker import FixedLocator
from matplotlib.dates import DateFormatter
import numpy as np
from datetime import datetime

# dates, irregularly spaced
y = 2010, 2010, 2010, 2011, 2011
m =    9,   11,   12,    3,    4


dates = np.array([plt.date2num(datetime(yy,mm,1)) for yy,mm in zip(y,m)])
print dates


# just some data
data = dates**2
print data


plt.plot_date(dates,data)

ax = plt.gca()
# need some more thnking for properly locate which date to label
#ax.xaxis.set_major_locator(FixedLocator(dates[[1, 10, len(dates)-1]]))
ax.xaxis.set_major_formatter(DateFormatter("%Y-%m"))
plt.show()

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