Trying to use matplotlib in a python CGI script naïvely will most likely result in the following error:
{{{
...
352, in _get_configdir
raise RuntimeError("'%s' is not a writable dir; you must set
environment variable HOME to be a writable dir "%h)
RuntimeError: '<WebServer DocumentRoot>' is not a writable dir; you must set
environment variable HOME to be a writable dir
}}}

Matplotlib needs the environment variable HOME to point to a writable directory. One way to accomplish this is to set this environment variable from within the CGI script on runtime (another way would be to modify the {{{.htaccess}}} file but that would be not as portable). The following template can be used for a cgi that uses matplotlib to create a png image:

{{{#!python
#!/usr/bin/python
import os,sys
import cgi
import cgitb; cgitb.enable()

# set HOME environment variable to a directory the httpd server can write to
os.environ[ 'HOME' ] = '/tmp/'

import matplotlib
# chose a non-GUI backend
matplotlib.use( 'Agg' )

import pylab

#Deals with inputing data into python from the html form
form = cgi.FieldStorage()

# construct your plot
pylab.plot([1,2,3])

print "Content-Type: image/png\n"

# save the plot as a png and output directly to webserver
pylab.savefig( sys.stdout, format='png' )
}}}

This image can then be accessed with a URL such as: http://localhost/showpng.py

As documented,some backends will not allow the output to be sent to sys.stdout. It is possible to replace the last line with the following to work around this: 
{{{#!python
pylab.savefig( "tempfile.png", format='png' )
import shutil
shutil.copyfileobj(open("tempfile.png",'rb'), sys.stdout)
}}}
(Of course it is necessary to create and delete proper temp files to use this in production.)
----
CategoryCookbookMatplotlib