Comment 1 for bug 301007

python-matplotlib 0.98.3-4ubuntu1 depends on "python-tk | python-gtk2 | python-wxgtk2.8 | python-qt3 | python-qt4". This means, that you need to have one of those packages installed. If you have none of those packages installed and you install python-matplotlib, python-tk will be installed. If you have one of those packages installed (except python-tk) and you install python-matplotlib, you have to install python-tk manually or change the default backend (in /etc/matplotlibrc).
You always need the python package installed wich correspond to the selected backend. I do not know how to assure that the correct python package is installed. The other way is to select the backend based on the installed packages. A plan for the future is to have a "auto" backend which searches for the installed packages and select the first he finds.