chart_get_scale (smax, smin, &lower, &interval, &ticks);
tick_format_string = chart_get_ticks_format (lower, interval, ticks, &tickscale);
upper = lower + interval * (ticks+1);
chart_get_scale (smax, smin, &lower, &interval, &ticks);
tick_format_string = chart_get_ticks_format (lower, interval, ticks, &tickscale);
upper = lower + interval * (ticks+1);