DateTimeAxis should also have "pan" and "zoom" properties, like ValueAxis

Use-case

A user should be able to pre-load 24 hours of data, then display 1 hour's worth of data in the GraphsView viewport, and scroll/pan across the 24 hours.

This is currently possible if we use ValueAxis (say, using indices instead of properly-formatted timestamps), but not possible if we use DateTimeAxis.