visibility of elements(rectangle) while using chartview in qml

[unknown user]

I'm using ChartView in QML to display a histogram, and I want to overlay a Rectangle on top of it to act as a discriminator line. However, the rectangle is hidden beneath the chart. I've tried setting a high z value on the Rectangle, but it still doesn't show up.
The strange part is that the rectangle becomes visible only after user interaction — such as a touch or a click on the screen. Until then, it's completely hidden.

Rectangle{
id: rectangle
color: "#1A1A1A"
anchors.fill: parent
radius: 5

ChartView {
    id:zoomedHistogram
    x: 0
    y: 0
    z: 0
    width: 100 + 400*1.25
    height: 394
    legend.visible: false
    antialiasing: true
   

    ValueAxis {
        id: axisX
        ..........................................
        ...................................................
    }

    ValueAxis {
        id: axisY
       ..........................................
       ...................................................
    }
    
    AreaSeries {
        axisY: axisY
       ..........................................
       ...................................................
        }
    }

}

Item{
    id : itemcombined
    z: 50

    Item{
        id: element1
        x: 56
        width: zoomedHistogram.d1/.256
        anchors.top: zoomedHistogram.bottom
        anchors.topMargin: 4
        height: 34
        z: 4
        y: zoomedHistogram.y + zoomedHistogram.height + 4

        Rectangle{
            id: colourRect
            color: ma.pressed ? "#2094FA" : "#3F3F3F"
            anchors.bottom: parent.bottom
            anchors.bottomMargin: 0
            anchors.top: parent.top
            anchors.topMargin: 0
            width: parent.width
        }
        Rectangle{
            id: colourLine
            color: ma.pressed ? "#2094FA" : "#363636"
            z: 200
            anchors.horizontalCenter: movingbutton.horizontalCenter
            anchors.bottomMargin: - 40
            width: 1
            height: zoomedHistogram.height + 5
            anchors.bottom: movingbutton.top

        }

        Rectangle{
            id: movingbutton
            height: 16
            width: 16
            radius: width/2
            anchors.verticalCenter: parent.verticalCenter
            x: parent.width
            color: "#000000"
            property var previous: 0
            onXChanged: {
            
                colourRect.width = x + 8
         
                if(previous>=movingbutton.x)
                    var asdf = Math.ceil(.256*movingbutton.x)
                else
                    asdf = parseInt(.256*movingbutton.x)
                previous = movingbutton.x
   
         
                zoomedHistogram.d1 = Number(asdf)
                if(ma.pressed){
                    window.onDraggedDescriminator()
                    window.totalSum()
                }
            }

            MouseArea{
                id: ma
                enabled: parameters.totalSumWBC>0 ? true : false
                anchors.fill: parent
                drag.target: parent
                drag.maximumX: zoomedHistogram.d2/.256 - 5
                drag.minimumX: 0
            }
            Rectangle{
                width: 5
                height: 5
                radius: width/2
                color: ma.pressed ? "#2094FA" : "#363636"
                anchors.centerIn: parent
            }
        }

    }

}