Qwt Plot开发：如何基于鼠标光标实现缩放功能？

Got it, let's fix this—by default QwtPlotMagnifier zooms around the plot center, but it's straightforward to modify it to zoom relative to where your mouse is pointing. Here's a step-by-step solution:

1. Create a Custom Magnifier Class #

We'll subclass QwtPlotMagnifier and override the rescale method to implement cursor-centered zoom logic:

#include <QwtPlotMagnifier>
#include <QwtPlot>
#include <QwtScaleMap>
#include <QCursor>

class CursorCenteredMagnifier : public QwtPlotMagnifier
{
public:
    explicit CursorCenteredMagnifier(QWidget* canvas)
        : QwtPlotMagnifier(canvas)
    {}

protected:
    void rescale(double factor) override
    {
        QwtPlot* plot = this->plot();
        if (!plot)
            return;

        // Get mouse position relative to the plot canvas
        QPoint mouseCanvasPos = canvas()->mapFromGlobal(QCursor::pos());
        // Fall back to default center zoom if mouse is outside the canvas
        if (!canvas()->rect().contains(mouseCanvasPos))
        {
            QwtPlotMagnifier::rescale(factor);
            return;
        }

        // Convert screen coordinates to plot data coordinates
        QwtScaleMap xMap = plot->canvasMap(QwtPlot::xBottom);
        QwtScaleMap yMap = plot->canvasMap(QwtPlot::yLeft);
        double mouseDataX = xMap.invTransform(mouseCanvasPos.x());
        double mouseDataY = yMap.invTransform(mouseCanvasPos.y());

        // Grab current axis ranges
        const QwtScaleDiv& xDiv = plot->axisScaleDiv(QwtPlot::xBottom);
        const QwtScaleDiv& yDiv = plot->axisScaleDiv(QwtPlot::yLeft);
        double xMin = xDiv.lowerBound();
        double xMax = xDiv.upperBound();
        double yMin = yDiv.lowerBound();
        double yMax = yDiv.upperBound();

        // Calculate new ranges: keep the mouse's data point fixed on screen
        double newXMin = mouseDataX - (mouseDataX - xMin) * factor;
        double newXMax = mouseDataX + (xMax - mouseDataX) * factor;
        double newYMin = mouseDataY - (mouseDataY - yMin) * factor;
        double newYMax = mouseDataY + (yMax - mouseDataY) * factor;

        // Apply the new scale to the plot
        plot->setAxisScale(QwtPlot::xBottom, newXMin, newXMax);
        plot->setAxisScale(QwtPlot::yLeft, newYMin, newYMax);
    }
};

2. Replace the Default Magnifier #

In your plot initialization code, swap out the default magnifier with our custom one:

// Remove the default magnifier (if it was automatically created)
delete ui->plot->magnifier();

// Install our cursor-centered magnifier
new CursorCenteredMagnifier(ui->plot->canvas());

Key Details to Note #

• Axis Support: This example uses xBottom and yLeft axes. If your plot uses other axes (like xTop or yRight), adjust the axis enum values in the code.
• Mouse Outside Canvas: We added a check to fall back to the default center zoom if the mouse is outside the plot canvas—this is a nice usability touch.
• Zoom Factor: The default wheel zoom factor is 2.0 (zoom in) and 0.5 (zoom out). You can adjust this with setWheelFactor(double factor) on your magnifier instance.
• Qwt Version: This code works with Qwt 6.x (the most widely used stable branch).

内容的提问来源于stack exchange，提问作者Süleyman Kabadayı