QCustomPlot add scatter points on top of existing plot - c++

Question

After creating a plot using qcustomplot, how can I, based on existing X values, retrieve the Y values, and then plot points at these locations?
My attempt is below:
Create plot:
This function creates a plot, adds some data (dates) on the x-axis and some values on the y axis:
void Qt_PlotTest::createPlot(){
QCustomPlot* customPlot = ui.widget;
customPlot->setLocale(QLocale(QLocale::English, QLocale::UnitedKingdom));
//Set some data:
double now = QDateTime::currentDateTime().toTime_t();
QVector<double> yData, xData;
for (int i = 0; i < 100; i++){
xData.push_back(now + i*24.0 * 3600.0);
yData.push_back(pow(double(i), 2) + 550.0*sin(double(i)/4.0));
}
// create graph and assign data to it:
customPlot->addGraph();
customPlot->graph(0)->setData(xData, yData);
//Fix axes:
//Set date axis:
QSharedPointer<QCPAxisTickerDateTime> dateTicker(new QCPAxisTickerDateTime);
dateTicker->setDateTimeFormat("d. MMMM\nyyyy");
customPlot->xAxis->setTicker(dateTicker);
customPlot->xAxis->setLabel("Time");
customPlot->yAxis->setLabel("Value");
customPlot->rescaleAxes();
customPlot->replot();
//Set interations:
customPlot->setInteraction(QCP::iRangeDrag, true);
customPlot->setInteraction(QCP::iRangeZoom, true);
}
Which results in:
Set scatter points:
This functions should plot a few scatter points at correct x/y locations based on x values. (I need to read the corresponding Y value from the existing plot).
void Qt_PlotTest::setScatterPoints(){
QCustomPlot* customPlot = ui.widget;
//The locations where scatter points should be plotted:
QDate qd1(2018, 03, 26), qd2(2018, 03, 30), qd3(2018, 05, 11), qd4(2018, 06, 15);
QVector<double> dates = { (double)QDateTime(qd1).toTime_t(), (double)QDateTime(qd2).toTime_t(), (double)QDateTime(qd3).toTime_t(), (double)QDateTime(qd4).toTime_t() };
//Use tracer to find data at these dates:
QVector<double> values;
QCPItemTracer *tracer = new QCPItemTracer(customPlot);
tracer->setGraph(customPlot->graph(0));
tracer->setInterpolating(true);
for (int i = 0; i < 4; i++){
tracer->setGraphKey(dates[i]);
values.push_back(tracer->position->coords().y());
}
//Plot points at these locations:
QCPGraph* dwPoints = new QCPGraph(customPlot->xAxis, customPlot->yAxis);
dwPoints->setAdaptiveSampling(false);
dwPoints->setLineStyle(QCPGraph::lsNone);
dwPoints->setScatterStyle(QCPScatterStyle::ssCircle);
dwPoints->setPen(QPen(QBrush(Qt::red), 2));
dwPoints->addData(dates, values);
customPlot->replot();
}
Which results in:
So obviously the QCPItemTracer didn't find the correct Y values. (I also got some additional axes which I don't want.)
Is QCPItemTracer what I want to use to achieve what I want? I also saw some example using QCPDataMap to find Y-values based on X-values. But as far as I understand, QCPDataMap, is no longer in qcustomplot.

Answer 1

I solved my own problem:
I had to call tracer->updatePosition() to actually get the coordinates in the coordinate system in which the tracer is placed.
The axes I didn't want were actually the visualization of the tracer at the final tracer location. I just had to call tracer->setVisibile(false) to hide it.
So in void Qt_PlotTest::setScatterPoints I have this:
QCPItemTracer *tracer = new QCPItemTracer(customPlot);
tracer->setGraph(customPlot->graph(0));
tracer->setInterpolating(true);
tracer->setVisible(false);
for (int i = 0; i < 4; i++){
tracer->setGraphKey(dates[i]);
tracer->updatePosition();
values.push_back(tracer->position->coords().y());
}
Result: