axis.x.extent doesn't behave properly when using a time series

When charting some simple data over a time series, `axis.x.extent` centres over _half_ of the first and last frames, rather than encompassing the specified range entirely.

Here's a [jsfiddle](http://jsfiddle.net/gg5n3mfr/) showing the example. Note that setting `axis.x.extent` to a two-integer list like `[3, 5]` does nothing. If you have a list of `Date` objects, it appears that you must specify those dates in the extent range. However, when you do, the little grey box that overlays your subchart straddles only half of a column.