Use .attr() instead of .style() for axis label's "text-anchor" assignment.

This allows the label's text-anchor setting to be overridden by CSS without having to resort to `!important`-statements (which are ugly and a maintenance hell) - for example to create horizontally centered labels on vertical axes. You are using `attr()` for visual attributes pretty much everywhere else in the code, so maybe there's some reason I've overlooked for specifically using `style()` here, but from what I see there should be no reason to not use `attr()` for assigning the `text-anchor` value.