QCustomPlot draw residuum between two functions - c++

Question

I want to draw the residuum between two functions at discrete points with qcustomplot.
I know the position (x), the starting value y.at(x) and the height.at(x).
what I have so far is an error bar with y+-error:
QCPErrorBars *errorBars = new QCPErrorBars(customPlot->xAxis, customPlot->yAxis);
errorBars->setDataPlottable(customPlot->graph(0));
QVector<double> y1err(x.size());
for (int i = 0; i<x.size(); ++i)
{
y1err[i] = y.at(i) * error;
}
customPlot->graph(0)->setData(QVector<double>::fromStdVector(x), QVector<double>::fromStdVector(y));
errorBars->setData(y1err);
or a bar starting from zero:
QCPBars *newBars = new QCPBars(customPlot->xAxis, customPlot->yAxis);
std::vector<double> xData, yData;
for (auto i = 0; i < x.size(); ++i)
{
xData.push_back(i+1);
yData.push_back(y.at(i));
}
newBars->setData(QVector<double>::fromStdVector(x), QVector<double>::fromStdVector(y));
but what I really want is some kind of a plot starting at the value y.at(x) with the height of the residuum at the point x in addition to the two x-y plots.
How can I plot a bar or error bar starting at the y.at(x) with height.at(x)?
Thank you

Answer 1

For other people facing this problem I found some kind of a solution.
void QLinePlot::AddResiduumData(std::vector<double> x, std::vector<double> y_mid, std::vector<double> y_res)
{
customPlot->addGraph();
++graphCountI;
QCPErrorBars *errorBars = new QCPErrorBars(customPlot->xAxis, customPlot->yAxis);
errorBars->setDataPlottable(customPlot->graph(graphCountI - 1));
customPlot->graph(graphCountI - 1)->setData(QVector<double>::fromStdVector(x), QVector<double>::fromStdVector(y_mid));
customPlot->graph(graphCountI - 1)->setVisible(false);
errorBars->setData(QVector<double>::fromStdVector(y_res));
customPlot->replot();
}
The idea behind this is adding a new invisible graph between both plots and give the error as half the distance between both.