Add Dotplot variant to plotter

Summary:
Adds a Dotplot to the plotter, which behaves somewhat like the "pic"
type, but always has dots. Also adds a label interval (show label every
'n' ticks), which is necessary for the dotplot to look reasonable.

{F13687}

Test Plan:
* Open test.html
* Play with the dot plot
* Try the other plotters to make sure they work as well

Reviewers: alex, jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7051