Arbitrary axis interval without affecting the plot

[Uwe]

In my case the x value of the points is just an array index (0-n).
If I keep it this way then the bottom axis shows me a range 0-n when I want it to be a time (I know that the points represents certain span of time, ie 20ms).
I know I could convert x value to time value but when zooming in/out I would like to be able to change the values displayed on the axis as it doesn't make much sense to show 0.00002ms, 20ns would be much nicer.

0.00002ms, 20ns are different representations of the same value.
Changing the tick values, when you want to change its representation only is somehow "von hinten durch die Brust ins Auge".

I'm fairly new to Qwt so I may be missing something obvious here.

See http://qwt.sourceforge.net/class_qwt...298de13483b61f.

Check the cpuplot example how to use it.

Uwe

[parasonic]

Thank you for your replies. My task is to create a clone for the Matlab imagesc() which has one form where one can specify the axis extents in the arguments.

Would the following code theoretically change the x-axis to range from -100 to 20? As it is, the code here still allows the x-axis to be drawn from 0 to 127, and the plot shows up compressed and on the right hand side.

class RD_Qwt_Plot : public QwtPlotSpectrogram
{
public:
    RD_Qwt_Plot()
    {
        setItemAttribute(QwtPlotItem::AutoScale, false);
    }
 
    ~RD_Qwt_Plot()
    {
    }
 
    virtual void draw(QPainter* painter, const QwtScaleMap &xMap,
        const QwtScaleMap &yMap, const QRectF &canvasRect) const
    {
        QwtScaleMap cust_x_map;
        cust_x_map.setPaintInterval(xMap.p1(), xMap.p2());
        // Matrix spans 128 for the X direction
//        cust_x_map.setScaleInterval(0, 127);
        cust_x_map.setScaleInterval(-100, 20);
 
        QwtScaleMap cust_y_map;
        cust_y_map.setPaintInterval(yMap.p1(), yMap.p2());
        // Matrix spans 16384 for the Y direction
        cust_y_map.setScaleInterval(0, 16383);
 
        QwtPlotSpectrogram::draw(painter, cust_x_map, cust_y_map, canvasRect);
 
    }
 
/*
    // This method is referenced when checking to see what data to be plotted.
    // If you set the interval here, it changes both the axis and the
    // data plotted.
    QwtInterval interval(Qt::Axis axis) const
    {
        const QwtInterval interv(-34, 67);
        return interv;
    }
*/
};

class RD_Qwt_Plot : public QwtPlotSpectrogram
{
public:
    RD_Qwt_Plot()
    {
        setItemAttribute(QwtPlotItem::AutoScale, false);
    }

    ~RD_Qwt_Plot()
    {
    }

    virtual void draw(QPainter* painter, const QwtScaleMap &xMap,
        const QwtScaleMap &yMap, const QRectF &canvasRect) const
    {
        QwtScaleMap cust_x_map;
        cust_x_map.setPaintInterval(xMap.p1(), xMap.p2());
        // Matrix spans 128 for the X direction
//        cust_x_map.setScaleInterval(0, 127);
        cust_x_map.setScaleInterval(-100, 20);

        QwtScaleMap cust_y_map;
        cust_y_map.setPaintInterval(yMap.p1(), yMap.p2());
        // Matrix spans 16384 for the Y direction
        cust_y_map.setScaleInterval(0, 16383);

        QwtPlotSpectrogram::draw(painter, cust_x_map, cust_y_map, canvasRect);

    }

/*
    // This method is referenced when checking to see what data to be plotted.
    // If you set the interval here, it changes both the axis and the
    // data plotted.
    QwtInterval interval(Qt::Axis axis) const
    {
        const QwtInterval interv(-34, 67);
        return interv;
    }
*/
};

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Here is a screen shot of what currently happens.
cap.jpg

Thanks

[Uwe]

My task is to create a clone for the Matlab imagesc() which has one form where one can specify the axis extents in the arguments.

See QwtPlot::setAxisScale() and check the spectrogram example how to eliminate the margins. If you want to limit the area, where to display your spectrogram see QwtRasterData::setInterval().

What the heck is your motivation to manipulate the canvas maps ?

Uwe