QML ChartView lineseries with "blank" points

[davidesalvetti]

Hi all,

I recently started working with qml and in particular with ChartView. I'm developing an app where I need to show a ChartView object with some LineSeries inside. I said "some LineSeries" but the end result should looks like a single line, the problem is that this single line has to be composed by different colours. It is certainly easier to understand with an image:

So, here I have two lines, but I would like them to look like one. To do this, I would like to know if there is a way to tell the QXYSeries Object to insert something like a blank point, so that I don't have the straight lines, because they're something that des not represent the line I want to draw. The correct line should look like the following one, but with two different color:

This is my current code:
MyChartView.qml

import QtQuick 2.15
import QtQuick.Controls 2.15
import QtCharts 2.15

ChartView {
    id: chartId

    plotArea: Qt.rect(5, 5, chartId.width - 5, chartId.height - 5)
    margins { top: 10; bottom: 10; left: 10; right: 10; }

    plotAreaColor: "#000020"
    backgroundColor: "white"
    legend.visible: false
    antialiasing: true

    ValueAxis {
        id: yaxis
        min: 0
        max: 100

        labelsVisible: false
        gridVisible: false
        minorGridVisible: false
        visible: false
    }

    ValueAxis {
        id: xaxis
        min: 0
        max: 100

        labelsVisible: false
        gridVisible: false
        minorGridVisible: false
        visible: false
    }

    LineSeries {
        id: firstId
        axisX: xaxis
        axisY: yaxis
        useOpenGL: true
        name: "firstId"
        color: "red"
    }

    LineSeries {
        id: secondId
        axisX: xaxis
        axisY: yaxis
        useOpenGL: true

        name: "secondId"
        color: "white"
    }
}

cpp

#include "chart.h"

Chart::Chart(QObject *parent) : BaseChart(parent)
{

}

void Chart::createDataPoint()
{
//this function popolates the firstId and secondId QVector<PointF>
    emit updateData();
}

void Chart::update(QAbstractSeries *series)
{
    if (series) {
        QXYSeries *xySeries = static_cast<QXYSeries *>(series);

        if (xySeries->name() == "firstId")
            xySeries->replace(firstId);
        else
            xySeries->replace(secondId);
    }
}

.h

#include <QObject>
#include <QtCharts/QXYSeries>
#include <QtCharts>
#include <QtCharts/QAbstractSeries>
#include "basechart.h"


class Chart: public BaseChart
{
    Q_OBJECT
public:
    explicit Chart(QObject *parent = nullptr);

    Q_INVOKABLE void update(QAbstractSeries *series);

    void createDataPoint();

signals:
    void updateData();
    void lineChanged();

private:
    QVector<QPointF> firstId;
    QVector<QPointF> secondId;


};

baseChart.cpp

#include <QObject>
#include <QtCharts>

QT_CHARTS_USE_NAMESPACE
Q_DECLARE_METATYPE(QAbstractSeries *)
Q_DECLARE_METATYPE(QAbstractAxis *)

namespace charts {
class BaseChart : public QObject
{
    Q_OBJECT
public:
    explicit BaseChart(QObject *parent = nullptr);


signals:

protected:
    

};
}

basechart.h

BaseChart::BaseChart(QObject *parent) : QObject(parent)
{
    qRegisterMetaType<QAbstractSeries*>();
    qRegisterMetaType<QAbstractAxis*>();
}

and from this qml I call the update on the series objects

import QtQuick 2.15
import QtQuick.Controls 2.15
import QtQuick.Layouts 1.15

Item {
    id: container
    anchors.fill: parent

    Connections {
        target: chart
        function onUpdateData() {
            console.log("We need to update data")
            chart.update(chartId.series(0))
            chart.update(chartId.series(1))
        }
    }

MyChartView {
            id: chartId
        }
}

I'm using Qt 5.15.2 on Windows 10 64bit with MinGw64.
If something is not clear please ask me! I had to delete some part of my code due to privacy policies but I hope it's clear what I would like to do.

The only solution I can think of right now is to create n-LineSeries object and assign to them the points and the color to display, but I would like to find an easier solution because this means that for my specific purpose I can reach more than 40-50 LineSeries to display on the chart. I would also need to create the LineSeries objects dinamycally from c++, but that should not be a problem.

Anyway, I know that in some charts libraries there is the possiblity to add something like a NoValue to avoid this behaviour, or there are some tricks, maybe also in Qt is there something similar?

Thanks in advance!

[davidesalvetti]

The solution I adopted in the end is to create dinamically a single LineSeries object for every line color, and that works pretty well even if I have to create 40-50 lineSeries objects.
I'm not pretty sure that the way I'm adding LineSeries is the best (I don't like the approach I used, but at the moment I don't see other possibilities).
When I need to draw the lines what I do is:

1. send a signal from c++ to qml that we need to draw new lines, with the number of lines to be drawn.
2. with a Connections object I intercept the signal in qml and inside a for I create a new LineSeries element using chartView.CreateSeries(). Then I pass the series back to c++, with the number of the LineSeries created, so that I know which points I have to assign the that LineSeries object.

I know that is not a clean approach, if somebody has any hints please tell me.

[davidesalvetti]

The solution I adopted in the end is to create dinamically a single LineSeries object for every line color, and that works pretty well even if I have to create 40-50 lineSeries objects.
I'm not pretty sure that the way I'm adding LineSeries is the best (I don't like the approach I used, but at the moment I don't see other possibilities).
When I need to draw the lines what I do is:

1. send a signal from c++ to qml that we need to draw new lines, with the number of lines to be drawn.
2. with a Connections object I intercept the signal in qml and inside a for I create a new LineSeries element using chartView.CreateSeries(). Then I pass the series back to c++, with the number of the LineSeries created, so that I know which points I have to assign the that LineSeries object.

I know that is not a clean approach, if somebody has any hints please tell me.