Added axis.focusFormat

The default ones are occasionally ill-suited for some graphs.  For
example, it shows months only on the ticks, and time only on hover.

Example:

Before:

![before](//img.skitch.com/20120831-d8s9fcci8633nnxpheikpff5nf.png)

After:

![after](//img.skitch.com/20120831-bbubrud6k5rqf8mh5mxddsc73w.png)