As mentioned in #232307 (specifically https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232307#c9) there is a necessity to have a matplotlib2 to not break dependencies. There is also a growing need to migrate to py3 and so a matplotlib3 port makes sense.
I would suggest *NOT* to add a new port right now.
After the default python version changed to 3.6 (ongoing work by antoine@), we could keep matplotlib 2.x as math/py-matplotlib2 (just for py27-only ports, e.g. science/py-hcluster) and update math/py-matplotlib to 3.x.