allow zooming for logscale plots * graphics.cc (axes::properties::zoom_about_point): Allow zooming for logscale axes. * graphics.h.in (class neg_log_scaler): New class. (log_scalar::do_neg_scale): Delete (log_scalar::scale): Don't attempt to handle negative data. (axes::properties::get_scale): New function. (axes::properties::update_xscale, axes::properties::update_yscale, axes::properties::update_zscale): Use it. (axes::properties::update_xlim): Call update_xscale. (axes::properties::update_ylim): Call update_yscale. (axes::properties::update_zlim): Call update_zscale.