Accessing Qt Charts with QDateTimeAxis in QML from C++

[54474N4]

I have a problem with Qt charts and with QDateTimeAxis. Inserted data won't display properly, when series is edited from C++ class. In main.qml I have only ChartView which is edited from C++ class DataHandler. DataHandler generates random data every second with current time stamp and edits the series in chart correspondigly. Also chart x-axis is edited dynamically. Changing the QDateTimeAxis to ValueAxis fixes the problem, but then the data time won't be in readable format in chart.

Here is main.cpp

#include <QGuiApplication>
#include <QApplication>
#include <QQmlApplicationEngine>
#include <QQmlEngine>
#include <QQmlContext>
#include <QQmlComponent>
#include "datahandler.h"

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

    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QApplication app(argc, argv);
    QQmlApplicationEngine engine;
 
    QQmlComponent component(&engine, QUrl(QLatin1String("qrc:/main.qml")));
    QObject *object = component.create();

    QScopedPointer<DataHandler> datahandler(new DataHandler(object));
    engine.rootContext()->setContextProperty("datahandler", datahandler.data());

    return app.exec();
}

Here is datahandler.h

#ifndef DATAHANDLER_H
#define DATAHANDLER_H
#include <QObject>
#include <QtCharts>

QT_CHARTS_USE_NAMESPACE

class DataHandler : public QObject
{
    Q_OBJECT

public:
    DataHandler(QObject* object, QObject *parent = 0);
    ~DataHandler();

private:
    QTimer* m_timer;
    QObject* m_object;
    QObject* m_chart;
    QVector<QPointF> m_data1, m_data2;

public slots:
    void addValues();

};

#endif // DATAHANDLER_H

...and datahandler.cpp

#include "datahandler.h"
#include <QObject>
#include <QTimer>
#include <QDateTime>
#include <QDebug>


DataHandler::DataHandler(QObject* object, QObject *parent)
{
    m_timer = new QTimer();
    m_object = object;
    connect(m_timer, &QTimer::timeout, this, &DataHandler::addValues);
    m_chart = m_object->findChild<QObject*>("chart");
    m_timer->start(1000);

}

DataHandler::~DataHandler(){

}

void DataHandler::addValues(){

    if(m_chart){
        //Generate the data
        m_data1.append(QPointF(QDateTime::currentMSecsSinceEpoch(), qrand() % 100));
        m_data2.append(QPointF(QDateTime::currentMSecsSinceEpoch(), qrand() % 100));
        QAbstractSeries *series1;
        QAbstractSeries *series2;
        QAbstractAxis *xAxis;
        //Fetch the series and x-axis from QML
        QMetaObject::invokeMethod(m_chart, "series", Qt::AutoConnection, Q_RETURN_ARG(QAbstractSeries*, series1), Q_ARG(int, 0));
        QMetaObject::invokeMethod(m_chart, "series", Qt::AutoConnection, Q_RETURN_ARG(QAbstractSeries*, series2), Q_ARG(int, 1));
        QMetaObject::invokeMethod(m_chart, "axisX", Qt::AutoConnection, Q_RETURN_ARG(QAbstractAxis*, xAxis));
        //Cast the series and axis to proper form
        QXYSeries* xyseries1 = static_cast<QXYSeries *>(series1);
        QXYSeries* xyseries2 = static_cast<QXYSeries *>(series2);
        QDateTimeAxis* dateTimeAxis = static_cast<QDateTimeAxis *>(xAxis);

        //set the new values to series and axises
        xyseries1->replace(m_data1);
        xyseries2->replace(m_data2);
        dateTimeAxis->setMax(QDateTime::fromMSecsSinceEpoch(m_data1.last().x()));
        dateTimeAxis->setMin(QDateTime::fromMSecsSinceEpoch(m_data1.first().x()));

    }
}

Here is main.qml

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.3
import QtCharts 2.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    ChartView{
        id: chart
        objectName: "chart"
        antialiasing: true
        legend.visible: false
        width: parent.width
        height: parent.height
        anchors.fill: parent
        theme: ChartView.ChartThemeBlueCerulean

        ValueAxis{
            id:axisY
            min:0
            max:100
        }
        DateTimeAxis{
            id:dateTime
            format: "hh:mm:ss"
            tickCount: 5
        }

        LineSeries{
            id:series1
            objectName: "series1"
            axisX: dateTime
            axisY: axisY
            useOpenGL: true
        }

        LineSeries{
            id: series2
            objectName: "series2"
            axisX: dateTime
            axisY: axisY
            useOpenGL: true
        }
    }
}

and ChartTest.pro file

QT += qml quick widgets charts

CONFIG += c++11

SOURCES += main.cpp \
    datahandler.cpp

RESOURCES += qml.qrc

# Additional import path used to resolve QML modules in Qt Creator's code model
QML_IMPORT_PATH =

# Additional import path used to resolve QML modules just for Qt Quick Designer
QML_DESIGNER_IMPORT_PATH =

# The following define makes your compiler emit warnings if you use
# any feature of Qt which as been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if you use deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

HEADERS += \
    datahandler.h

[54474N4]

I got the program working, by deleting and generating new series every time the chart update is required. First I delete and generate new series in QML and then call the addValues() function in C++ side. This solves the problem, but does not change the fact that there seems to be something wrong with the QDateTimeSeries update.

[54474N4]

I noticed that the actual cause of the problem is in LineSeries useOpenGL property. When OpenGL is on, the series won't update properly. After turnign OpenGL off, everything works just fine.

[54474N4]

@54474N4
There seems to be QT bug report about the issue:
https://bugreports.qt.io/browse/QTBUG-59674?jql=text ~ "OpenGL double float"