"Love your avatar".
Thanks Rod. It's a GIF re-processed by an online GIF editor that can make it go backwards.
Like this one:


Installing Mathmap.
The final thing I did was to make a symbolic link.
cd ~/.gimp-2.8/plug-ins/; ln -s /usr/bin/mathmap

After that Mathmap is not 'greyed-out' and should work. I originally followed YAFU's instructions.