I fill the color between the two graphs, both the section below the mRefGraph and the part above are filled

mGraph    = ui->plot->addGraph(ui->plot->xAxis2, ui->plot->yAxis);
mRefGraph = ui->plot->addGraph(ui->plot->xAxis2, ui->plot->yAxis);

mRefGraph->setBrush(QBrush(QColor(153, 53, 53, 94)));
mRefGraph->setChannelFillGraph(mGraph);

how can I fill only the lower area of the mRefGraph , not the upper area?