QChart unresponsive with large data set

Question

I have this code which works well for data sizes up to 1000. Now I tested it with 65536 points.

series = new QLineSeries();

QList<QPointF> points;
points.reserve(data.size());

for(std::vector<int>::size_type i = 0; i != data.size(); i++) {
    QPointF point(i, data[i]*100/max);
    points.append(point);
}
series->clear();
series->append(points);

And the application freezes with 1 core at full power. I stopped it after minutes.

How can I prevent Qt from becoming unresponsive. This data size is not special, I would expect a chart view to handle data sets up to million points.

EDIT: I measured the time

series->append(points);

takes 1 second for 2000 points. That means for about a minute for > 50.000 That is unusable.

Even worse, the log scale plot

serieslog->append(points);

takes 40 seconds for 2000 points. That is completely unusable. The reasons is the debug message, which is printed out for almost every point.

QtCharts::XLogYDomain::calculateGeometryPoints(const QVector&) const>; Logarithms of zero and negative values are undefined.

I can speed up the linear plot with

 series->setUseOpenGL(true);

However with 65536 it still takes 14 seconds, that means 200 µs per point. Still to much. I want a live video with 10 Hz minimum, and a live histogramm. the time must << 1 second.

EDIT: Here a working example, using my code

#include <QDebug>
#include <QTime>
#include <cmath>
#include <stdlib.h>

#include <QtCharts/QChartView>
#include <QtCharts/QLineSeries>
#include <QtCharts/QLogValueAxis>
#include <QtCharts/QValueAxis>
#include <QtWidgets/QApplication>
#include <QtWidgets/QMainWindow>

QT_CHARTS_USE_NAMESPACE

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QLineSeries * series;
    QLineSeries * serieslog;
    QChart * chart;
    QChartView * chartView;
    QValueAxis * axisX;
    QValueAxis * axisY;
    QLogValueAxis * axisY3;


    chart = new QChart();
    chart->legend()->hide();
    chart->setTitle("Histogramm");

    axisX = new QValueAxis;
    chart->addAxis(axisX, Qt::AlignBottom);

    series = new QLineSeries;
    chart->addSeries(series);

    axisY = new QValueAxis;
    axisY->setTitleText("linear scale");
    axisY->setLinePenColor(series->pen().color());
    axisY->setGridLinePen((series->pen()));

    chart->addAxis(axisY, Qt::AlignLeft);
    series->attachAxis(axisX);
    series->attachAxis(axisY);

    serieslog = new QLineSeries;
    chart->addSeries(serieslog);

    axisY3 = new QLogValueAxis();
    axisY3->setTitleText("logarithmic scale");
    axisY3->setLabelFormat("%g");
    axisY3->setLinePenColor(serieslog->pen().color());
    axisY3->setGridLinePen((serieslog->pen()));
    axisY3->setMinorTickCount(-1);

    chart->addAxis(axisY3, Qt::AlignRight);
    serieslog->attachAxis(axisX);
    serieslog->attachAxis(axisY3);

    chartView = new QChartView(chart);
    chartView->setRenderHint(QPainter::Antialiasing);

    // create data

    std::vector<int> data;
    int N = 10000;
    data.resize(N);
    for (int i=0; i < N; ++i){
        int value = static_cast<int>(fabs((sin(static_cast<double>(i)/1000.0)+1)*1+ std::rand() % 100)+10);
        data[i] = value;
    }

    QList<QPointF> points;
    points.reserve(data.size());

    for(std::vector<int>::size_type i = 0; i != data.size(); i++) { //
        QPointF point(i, data[i]);
        points.append(point);
    }
    QTime myTimer;
    myTimer.start();

    series->clear();
//    series->setUseOpenGL(true);
    series->append(points);
    qDebug() << "seconds lin: " << myTimer.elapsed();
    myTimer.start();
    serieslog->clear();
    serieslog->append(points);
    qDebug() << "seconds log: " << myTimer.elapsed();

    chart->axisX()->setRange(0, data.size());
    chart->axisY()->setRange(-10, 250);

    QMainWindow window;
    window.setCentralWidget(chartView);
    window.resize(800, 600);
    window.show();

    return app.exec();
}


QT += core
QT += widgets
QT += gui
QT += charts

SOURCES += \
    main.cpp

I measure mseconds lin: 1624 mseconds log: 6801

Answer 1

I can repro the problem (with similar elapsed times) and it appears to be an issue with the way QXYSeries::append handles QList. From the code...

void QXYSeries::append(const QList<QPointF> &points)
{
    foreach (const QPointF &point , points)
        append(point);
}

and...

void QXYSeries::append(const QPointF &point)
{
    Q_D(QXYSeries);

    if (isValidValue(point)) {
        d->m_points << point;
        emit pointAdded(d->m_points.count() - 1);
    }
}

So each point addition will potentially result in the QVector d->m_points being resized and the pointAdded signal being emitted.

Given that you clear all data associated with the series before calling QXYSeries::append you could use QXYSeries::replace instead.

If you must generate your initial data as a QList then just use...

series->replace(points);

However, internally that uses QList::toVector so if you can generate the data as a QVector then so much the better...

QVector<QPointF> points(data.size());

for(std::vector<int>::size_type i = 0; i != data.size(); ++i) {
  points[i] = QPointF(i, data[i]);
}

QTime myTimer;
myTimer.start();

series->replace(points);
qDebug() << "\nlin: " << myTimer.elapsed() << "ms\n";

myTimer.start();
serieslog->replace(points);
qDebug() << "\nlog: " << myTimer.elapsed() << "ms\n";

The above code on my own system results in...

lin:  1 ms
log:  3 ms

for 10k points, and for 100k points...

lin:  6 ms
log:  22 ms

Answer 2

I noticed this slowness as well, in my case I cleared and replaced the data in the series. I am using QML.

I got significant performance improvement when I set the series to be invisible before clearing and appending new data, and set it back to visible after I set the min and max etc.