Real-Time Plotting several graphs, using qtquick

[Sina Ranjkesh zade]

Hi,
I'm using qtquick to plot several graphs. There is some buttons which can I select and switch between these graphs to see the result. Graphs are being updated. When the length of the graphs gets more, It takes time to see the result when I switch between them and the plots get dark. Here is my code:

import QtQuick 2.0
import QtCharts 2.3

Item {

    id: root
    visible: true
    property int objectId: 0

    anchors.fill: parent

    property var upPoints
    property bool dataLoaded: false;

    property real distance
    property real minOfChart
    property real scaleFactor: (height/distance)
    property real offset: (height/distance)*(-minOfChart)

    property var listOfObjects: []

    Canvas {
        id: canvas

        anchors.fill: parent

        renderStrategy:Canvas.Threaded
        renderTarget:Canvas.FramebufferObject
        antialiasing: true
        smooth: true
        Rectangle {
            anchors.fill: parent
            color: "#656565"
            z:-1
        }

        onPaint: {

            if(dataLoaded == true)
            {
                var ctxU = getContext("2d");
                ctxU.reset();

                var lines = getContext("2d");

                ctxU.strokeStyle  = "#ffffff"
                ctxU.lineWidth  = 2;
                ctxU.translate(0,height)
                ctxU.scale(1,-1);

                ctxU.beginPath();
                ctxU.moveTo(0,0);

                for(var i=0;i<upPoints.length;i++)
                {
                    ctxU.lineTo(upPoints[i].x,upPoints[i].y*scaleFactor+offset);
                }
                ctxU.stroke();
            }

        }

        Connections {
            target: back

            onSendVedioData: {
                if(itemNumber == objectId )
                {

                    upPoints = data.upList;
                    dataLoaded = true;
                    distance = max - min
                    minOfChart = min;
                    canvas.requestPaint();
                }
            }
        }


    }
    function drawLines(contex)
    {
        contex.strokeStyle  = "#717171"
        contex.lineWidth  = 2;
        contex.beginPath();
        contex.moveTo(0,0);
        contex.lineTo(100,100);
        contex.stroke();
    }
}

You can watch a video in this link indicating the problem. As you can see, when I switch between "brightness" and "contrast", it gets dark.
https://drive.google.com/file/d/1_8NoF9_EE6bf5UhOddvjHl9gDkOT6Fcu/view?usp=sharing
Any help will be appreciated.

[Sina Ranjkesh zade]

@Sina-Ranjkesh-zade
As I guess, It's because whenever I change the feature from brightness to cotrast or vice versa, It plots the diagram from the beginning and because of the increasing number of points, It gets time to plot the diagram, so it gets dark.
If this guess is true, a solution is that I should save and load diagrams whenever I change the feature, but I don't know how can I do that?
Does anyone know?
Does anyone have better solution?
Any help will be appreciated.