Speed up plotting using numpy

[adsglass]

Just replaced the slow TODOs with some optimized numpy operations to speed up the plotting.

For a group size of 4 on the little snippet of code before the if ci..., the time taken on the loop improves from 248 ms ± 3.14 ms per loop to 960 µs ± 14.2 µs per loop. All unit tests pass.

[coveralls]

Coverage remained the same at 94.988% when pulling 8a4905e on adsglass:speed-up-plotting into 683e08c on better:master.

[erikbern]

nice! thanks @adsglass