As of pip 1.5 (released January 1, 2014), pip will no longer install externally hosted files by default.

If a user tries to pip install matplotlib it will default to the most recent package hosted on PyPI (1.1.1).

Pinning the dependency to install matplotlib 1.3.1 results in a helpful error message

$ pip install matplotlib==1.3.1
Downloading/unpacking matplotlib==1.3.1
  Could not find a version that satisfies the requirement matplotlib==1.3.1 (from versions: 0.86.1, 0.86.2, 0.86, 0.91.0, 0.91.1, 1.0.1, 1.1.0, 1.1.1)
  Some externally hosted files were ignored (use --allow-external to allow).
Cleaning up...
No distributions matching the version for matplotlib==1.3.1
Storing debug log for failure in /home/vagrant/.pip/pip.log

Users can work around this by either specifying to allow external and unverified,

$ pip install matplotlib==1.3.1 --allow-external matplotlib --allow-unverified matplotlib
Downloading/unpacking matplotlib==1.3.1
  matplotlib an externally hosted file and may be unreliable
  matplotlib is potentially insecure and unverifiable.
  Downloading matplotlib-1.3.1.tar.gz (42.2MB): 42.2MB downloaded
...

or by installing straight from the package for their respective system (tarball for *nix shown below)

$ pip install https://downloads.sourceforge.net/project/matplotlib/matplotlib/matplotlib-1.3.1/matplotlib-1.3.1.tar.gz

It would be really nice to have this package available for pip. It seems that in the past this was a filesize limit restriction workaround on PyPI, which can now handle bigger packages.

If the filesize is still an issue, perhaps we can work with the PyPI folks to either increase the limit or put matplotlib on a whitelist.