Qt繪圖控件qwt繪製等比例座標圖

須要用到QwtPlotRescaler類，用法以下：

QwtPlotRescaler *plotRescaler = new QwtPlotRescaler(canvas, yLeft, QwtPlotRescaler::Fitting ); 

plotRescaler->setExpandingDirection(QwtPlotRescaler::ExpandBoth);

其構造函數能夠設置參考座標軸，及其縮放動做，默認其餘軸與參考軸的比例爲1.0。setExpandingDirection函數能夠指定在固定好參考軸後，另外一軸拉伸時的模式，QwtPlotRescaler::ExpandBoth參數是指向兩邊擴展。

在找到這個類前，我有一個手動寫法，原理是將QwtPlotZoomer的zoomed(const QRectF &)信號與在它所在的QwtPlot類中編寫的槽相連，並重寫QwtPlot類中的resizeEvent事件：

void TrackPlot::zoomedSlot(const QRectF &rectF)
{
    QPointF centerPoint = rectF.center();

    double Ratio = rectF.width() / rectF.height();
    double hvRatio =  double(canvas()->width()) / double(canvas()->height());    

    if(Ratio > 1)
    {
        double scaleFactor = hvRatio / Ratio > 1 ? hvRatio / Ratio : 1;

        setAxisScale(xBottom, centerPoint.x() - scaleFactor * rectF.width() / 2.0, 
            centerPoint.x() + scaleFactor * rectF.width() / 2.0);
        double step = axisStepSize(xBottom);
        setAxisScale(yLeft, centerPoint.y() - scaleFactor * rectF.width() / (2.0 * hvRatio), 
            scaleFactor * centerPoint.y() + scaleFactor * rectF.width() / (2.0 * hvRatio), step);    
    }
    else
    {
        double scaleFactor = Ratio / hvRatio > 1 ? Ratio / hvRatio : 1;

        setAxisScale(yLeft, centerPoint.y() - scaleFactor * rectF.height() / 2.0, 
            centerPoint.y() + scaleFactor * rectF.height() / 2.0);
        double step = axisStepSize(yLeft);
        setAxisScale(xBottom, centerPoint.x() - scaleFactor * rectF.height() * hvRatio / 2.0, 
            centerPoint.x() + scaleFactor * rectF.height() * hvRatio / 2.0, step);
    }

    replot();
}

void TrackPlot::resizeEvent(QResizeEvent *ev)
{
    QwtPlot::resizeEvent(ev);

    if(ev->oldSize().width() > 5)
    {
        double hRatio = double(ev->size().width()) / double(ev->oldSize().width());
        double vRatio = double(ev->size().height()) / double(ev->oldSize().height());

        if(hRatio != 1)
            printf("");

        double xLowBound = axisScaleDiv(xBottom).lowerBound();
        double xUpperBound = axisScaleDiv(xBottom).upperBound();
        double yLowBound = axisScaleDiv(yLeft).lowerBound();
        double yUpperBound = axisScaleDiv(yLeft).upperBound();
        
        setAxisScale(xBottom, hRatio * xLowBound, hRatio * xUpperBound);
        double ystep = axisStepSize(xBottom);
        setAxisScale(yLeft, vRatio * yLowBound, vRatio * yUpperBound, ystep);

        replot();
    }
    else
    {
        double minX = axisScaleDiv(xBottom).lowerBound();
        double maxX = axisScaleDiv(xBottom).upperBound();
        double minY = axisScaleDiv(yLeft).lowerBound();
        double maxY = axisScaleDiv(yLeft).upperBound();

        QRectF rectF = QRectF(minX, minY, maxX - minX, maxY - minY); 

        QPointF centerPoint = rectF.center();

        double Ratio = rectF.width() / rectF.height();
        double hvRatio = double(canvas()->width()) / double(canvas()->height());    

        if(Ratio > 1)
        {
            double scaleFactor = hvRatio / Ratio > 1 ? hvRatio / Ratio : 1;

            setAxisScale(xBottom, centerPoint.x() - scaleFactor * rectF.width() / 2.0, 
                centerPoint.x() + scaleFactor * rectF.width() / 2.0);
            double step = axisStepSize(xBottom);
            setAxisScale(yLeft, centerPoint.y() - scaleFactor * rectF.width() / (2.0 * hvRatio), 
                scaleFactor * centerPoint.y() + scaleFactor * rectF.width() / (2.0 * hvRatio), step);    
        }
        else
        {
            double scaleFactor = Ratio / hvRatio > 1 ? Ratio / hvRatio : 1;

            setAxisScale(yLeft, centerPoint.y() - scaleFactor * rectF.height() / 2.0, 
                centerPoint.y() + scaleFactor * rectF.height() / 2.0);
            double step = axisStepSize(yLeft);
            setAxisScale(xBottom, centerPoint.x() - scaleFactor * rectF.height() * hvRatio / 2.0, 
                centerPoint.x() + scaleFactor * rectF.height() * hvRatio / 2.0, step);
        }

        replot();
    }
}