Add a few comments in the _do_zoom() method.
Add a few comments in the _do_zoom() method.

burnpanck/chaco,tommy-u/chaco,ContinuumIO/chaco,tommy-u/chaco,tommy-u/chaco,burnpanck/chaco,ContinuumIO/chaco,burnpanck/chaco,ContinuumIO/chaco,ContinuumIO/chaco