The lack of a python3-matplotlib package in wheezy is unfortunate.
However pip is usually an easy alternative to get your package installed.
When you get a gcc exit status 1 upon
sudo pip3 install matplotlib
it is telling you the build failed. You should check the output above what the exact error is.
Sometimes the error is clear and descriptive (not enough space on SD card for example). Even if the error text doesn't give you a clue, google might offer a simple fix.
I was in the exact same situation, gcc exit status 1, but above that: No space left on device. My /tmp directory is mounted in the memory and was sized to small. I imagine I could have increase the size in memory (editing the fstab file) but I choose to mount the /tmp folder temporary on a USB stick to be certain I have enough space.