plot.js: switch from jQuery map to underscore map

The former must be recursive, b/c it was causing stack frame errors in Safari.

Also, I no longer need to wrap the array -- apparently, underscore.js uses a proper
map where if you return an array it actually uses that as the element.
plot.js: do not for loop over jquery objects

No, really.

This makes the relevant function faster by a factor of ~4000+.
plot.js: simplify