QwtPlot axis labels

[Uwe]

The best solution should be to translate the x coordinates from pcm to milliseconds. As you probably don't want to copy your samples for this I recommend to derive from QwtSeriesData<QwtPointF> and do the translation there. F.e. something like this:

class YourSeriesData: public QwtSeriesData<QwtPointF>
{
        virtual QPointF sample( size_t i ) const
        {
            // depends how you have stored your samples
 
            double xPCM = ...;
            double y = ...;
 
            return QPointF( toMS( xPCM ), y );
        }
};

class YourSeriesData: public QwtSeriesData<QwtPointF>
{
        virtual QPointF sample( size_t i ) const
        {
            // depends how you have stored your samples

            double xPCM = ...;
            double y = ...;

            return QPointF( toMS( xPCM ), y );
        }
};

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Guess overloading YourSeriesData::boundingRect makes also sense as the x-range can be found easily for a series with increasing x values.

HTH,
Uwe

PS: It's always a good idea to check the cpuplot example - even if completely unrelated when you don't want to display the tick labels in a date/time format

[Kutuska]

I just want the label text to show another number and dont show me 50 000 when the value is still 50 000. Can I do that in qwt? Just to change the label's "text", the numbers only. I want the plot's values to represent the pcm data because its only working that way but show in label text the ms.

[Uwe]

Then you have to study the cpuplot example

In fact all you have to do is to derive from QwtScaleDraw and overload QwtScaleDraw::label(), where you add your pcmToMS conversion. But note that in opposite to my suggestions the scale engine calculates a scale based on pcm not on ms.

Uwe

[Kutuska]

I have this code. I think this has to work in theory. It compiles and runs but no change in text. Whats the problem in my code? If i qDebug the label's text at p it gives me the correct value but after I set the text of label(p) it won't change. Why?

QwtScaleDraw *xscale = ui->qwtPlot1->axisScaleDraw(QwtPlot::xBottom);
    QString text;
    for(int p= 1; p < lenPcm; p++){
        text.setNum(p/freq);
        xscale->label(p).setText(text);
    }
    ui->qwtPlot1->setAxisScaleDraw(QwtPlot::xBottom,xscale);
    ui->qwtPlot1->replot();

QwtScaleDraw *xscale = ui->qwtPlot1->axisScaleDraw(QwtPlot::xBottom);
    QString text;
    for(int p= 1; p < lenPcm; p++){
        text.setNum(p/freq);
        xscale->label(p).setText(text);
    }
    ui->qwtPlot1->setAxisScaleDraw(QwtPlot::xBottom,xscale);
    ui->qwtPlot1->replot();

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Thanks in advance!

[Uwe]

This is your code:

class YourScaleDraw: public QwtScaleDraw
{
public:
    virtual QwtText label( double v ) const
    {
        return QwtScaleDraw::label( pcmToMS( v ) );
    }
};
 
plot->setAxisScaleDraw( ..., new YourScaleDraw() );

class YourScaleDraw: public QwtScaleDraw
{
public:
    virtual QwtText label( double v ) const
    {
        return QwtScaleDraw::label( pcmToMS( v ) );
    }
};

plot->setAxisScaleDraw( ..., new YourScaleDraw() );

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If you want to have ticks aligned to ms values read my first answer or set your ticks manually ( what is no option if you want to offer a navigation like zooming/panning to your users ) with QwtPlot::setAxisScaleDiv().

HTH,
Uwe

[Kutuska]

Thanks a lot! Its working now!
Another quick question!
When I load a bigger file (about 3,5 mins or bigger) the Qt crashes and I think it's because qwtplot replots so much. It has to plot about 6 million dots. Its Qt's fault or there's a solution? And when I want to get a playback position indicator cursor (a vertical line) for my plot, it replots so much the Qt can't do it. Can I somehow cache the plot and then do the cursor over only the "picture" of the plot?

[Uwe]

When I load a bigger file (about 3,5 mins or bigger) the Qt crashes and I think it's because qwtplot replots so much.

Start your debugger and check what's going on. A crash is never because the CPU is busy - it's more likely that you running out of memory.

And when I want to get a playback position indicator cursor (a vertical line) for my plot, it replots so much the Qt can't do it. Can I somehow cache the plot and then do the cursor over only the "picture" of the plot?

Sounds like QwtPlotPicker is what you are looking for. Check the bode example ( press the mouse on the canvas ). Instead of a cross hair you can also have a vertical or horizontal line.

When you try to plot many many dots you might want to try Qwt from SVN trunk. It reintroduces some optimizations like:

• QwtPlotCurve::FilterPoints
• QwtPlotCurve::MinimizeMemory

Especially the first flag should speed up replotting in your situation a lot. But of course the best solution is to filter out points in application code before replotting !

Uwe