Make plot endpoint

[scottsfarley93]

Make a /plot endpoint that accepts two variable id/source id combinations and returns an x/y combination for each spatiotemporal point. Would significantly simplify the plotting code on the client.

[scottsfarley93]

Good, returns x,t,y tuples. But slow.

[scottsfarley93]

Won't return multiple tables if x and y source/variable combinations are the same.