QChartView/QChart custom Zoom In/Out

[Joe von Habsburg]

hello, i created a custom QChartView class and i want to perform zoom in/out operations on it. unfortunately i didn't get the result i wanted. when i do the zoom operation, it zooms according to the center of the chart. what i want is to apply the zoom operation according to the mouse position. how can i do this?

---

Chart::Chart(QGraphicsItem *parent, Qt::WindowFlags wFlags) : QChart(parent, wFlags)
{
    qDebug() << "Chart::Chart()";
    legend()->hide();
    setMargins(QMargins(1,1,1,1));
}

Chart::~Chart()
{
    qDebug() << "Chart::~Chart()";
}

---

ChartView::ChartView(QWidget *parent) : QChartView(parent)
{
    qDebug() << "ChartView::ChartView()";
    setMouseTracking(true);
    setDragMode(QGraphicsView::ScrollHandDrag);
    setStyleSheet("background-color:white");
}

ChartView::~ChartView()
{
    qDebug() << "ChartView::~ChartView()";
}

void ChartView::wheelEvent(QWheelEvent *event)
{
    qDebug() << "ChartView::wheelEvent";

    if (event->angleDelta().y() > 0) {
        chart()->zoom(1.05); // Zoom In
    } else {
        chart()->zoom(1 / 1.05); // Zoom Out
    }

    event->accept();
}

[JonB]

@Joe-von-Habsburg
I don't know about QChartView but I presume it behaves the same as QGraphicsView. And for that you need to use setTransformationAnchor(AnchorUnderMouse) prior to scale().

FWIW, I paste the code I am using in some code of mine.

/*virtual*/ void BaizeView::wheelEvent(QWheelEvent *event) /*override*/
{
    // Do a wheel-based zoom about the cursor position
    // (Adapted from https://stackoverflow.com/questions/19113532/qgraphicsview-zooming-in-and-out-under-mouse-position-using-mouse-wheel)
    event->accept();

    int zoomBy = event->angleDelta().y() / 120;
    qreal currentZoom = transform().m11();
    qreal factor;
    if (zoomBy > 0 && currentZoom < 3.0)
        factor = 1.1;
    else if (zoomBy < 0 && currentZoom > 0.333)
        factor = 1.0 / 1.1;
    else
        return;

    if (false)
    {
        // moves scroll position to odd place
        const ViewportAnchor anchor = transformationAnchor();
        setTransformationAnchor(AnchorUnderMouse);
        scale(factor, factor);
        setTransformationAnchor(anchor);
    }
    else
    {
        // seems to get it right
        auto targetViewportPos = event->position();
        auto targetScenePos = mapToScene(event->position().toPoint());
        scale(factor, factor);
        centerOn(targetScenePos);
        QPointF deltaViewportPos = targetViewportPos - QPointF(viewport()->width() / 2.0, viewport()->height() / 2.0);
        QPointF viewportCenter = mapFromScene(targetScenePos) - deltaViewportPos;
        centerOn(mapToScene(viewportCenter.toPoint()));
    }
    emit viewCoordinatesChanged();
}

It may be a bit different from what you want, I am maintaining a scroll position on my QGraphicsView which may not apply to you. So you should try the if (false) route first.

For QChartView I note the following very old post: https://interest.qt-project.narkive.com/5wgzvCSD/handling-zooming-in-qtcharts. That confirms:

Because QChart is derived from QGraphicsView, you can also set the
setTransformationAnchor() to
QGraphicsView::AnchorUnderMouse and then scale() it.

But if that is not right the OP there ended up saying:

I was able to find a way to zoom centred on the mouse. What I did was
make use of the QChart::plotArea() function. With that function by
using ChartView::wheelEvent(), I could get where in the chart the zoom
is happening. I then did some math and set the new zoom for the chart
by adjusting the range for the chart's x and y axis.

[Joe von Habsburg]

@JonB said in QChartView/QChart custom Zoom In/Out:

For QChartView I note the following very old post: https://interest.qt-project.narkive.com/5wgzvCSD/handling-zooming-in-qtcharts. That confirms:

there is a problem here as there is with me.

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@JonB said in QChartView/QChart custom Zoom In/Out:

I don't know about QChartView but I presume it behaves the same as QGraphicsView.

Is it possible to draw lineseries without using qchart and qchartview? if possible, how can it be done?

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@JonB said in QChartView/QChart custom Zoom In/Out:

FWIW, I paste the code I am using in some code of mine.

I added to my code. here it behaves like a picture. shouldn't the axes be fixed?

Zoom Out

Zoom In

[JonB]

@Joe-von-Habsburg
Yes you can draw lines on a QGraphicsScene, I never suggested you not use a QChart (or equivalent, QChart is not considered wonderful and it is being replaced by QtGraphs, for all I know that may be better here) for drawing charts, there is a reason that is its own component.

Since I don't know anything about QCharts and zooming I would not know about axes and what the consequences might be. I only know what I wrote and the links I referred you to, which may have alternative approaches.