if ($graph_params->visible('previous')) {
    if (is_numeric($multiplier)) {
        $rrd_options .= ' CDEF:X=' . $thingX . $plusesX . ',' . $multiplier . ',*';
    } elseif (is_numeric($divider)) {
        $rrd_options .= ' CDEF:X=' . $thingX . $plusesX . ',' . $divider . ',/';