How to draw graph for a trajectory which goes left and right in x axis?

i want to draw a trajectory in x and y of a car in a parking lot.

the trajectory in x is not always in the same direction. sometime the car will go left.

the problem here is: sometime (not always!) the graph will no go left in x axis. You can see the two different result on the image https://i.sstatic.net/a0yDx.jpg

any idea why?

the image at left is what i expect. at right is the same values , but i continue to plot data a little longer.

void TrackingResultsView::setupTrajectoryPlot()
{
QCustomPlot *customPlot = ui->qcp_trajectory;

customPlot->xAxis2->setVisible(true);
customPlot->xAxis2->setLabel("X-Position (pixel)");
customPlot->xAxis2->setRange(0, mModelPtr->frameSize().width());
customPlot->xAxis2->grid()->setVisible(true);

customPlot->xAxis->setRange(0, mModelPtr->frameSize().width());
customPlot->yAxis->setLabel("Y-Position (pixel)");
customPlot->yAxis->setRange(0, mModelPtr->frameSize().height());
customPlot->yAxis->setRangeReversed(true);

customPlot->yAxis2->setVisible(true);
customPlot->yAxis2->setRange(0, mModelPtr->frameSize().height());
customPlot->yAxis2->grid()->setVisible(true);
customPlot->yAxis2->setRangeReversed(true);
customPlot->addGraph(customPlot->xAxis2, customPlot->yAxis);

QVector<QVector<double>> data = createDataMap(mModelPtr->points());
customPlot->graph()->setData(data.at(0), data.at(1), true);

setTheme(customPlot, false);
}

thank you

(english is not my first langage)

Solution

The QCPGraph seems to be used for sorted data with only value per key. From QCustomPlot documentation, it looks like the QCPCurve would be a better match in order to plot a trajectory graph (multiple value for the same key).

From the QCPCurve description:

Unlike QCPGraph, plottables of this type may have multiple points with the same key coordinate, so their visual representation can have loops. This is realized by introducing a third coordinate t, which defines the order of the points described by the other two coordinates x and y.