Only plot a point in range selector if it changes the x-value

This is a variation on #446.

Compare [before](https://cdn.rawgit.com/danvk/dygraphs/cd6172e1a27775a42da61723b324236b1f461cf6/tests/range-selector.html) and [after](https://cdn.rawgit.com/danvk/dygraphs/range-selector-granularity/tests/range-selector.html). There's a slight change to appearance which I am comfortable with. Ideally we'd plot only the highest point that maps to each integral x-value, but that's a larger change.

Fixes #399