Overhaul __plt_get_axis_arg__ and newplot functions to avoid creating unnecessary axes. * scripts/plot/__plt_get_axis_arg__.m: Only return an axis handle if found in argument list. Do not create any figures or axes. * scripts/plot/newplot.m: Overhaul to allow specifying an axis handle input. Prepare figure and axes according to Matlab conventions. * scripts/plot/line.m: Never call newplot() for a core graphic object. Always plot into gca. * scripts/plot/plot.m: Update to use new __plt_get_axis_arg__ and newplot functions.