Labels on QwtPlot

[penny]

I'm overloading the drawCanvas method to display text on my QwtPlotHistogram. However when i zoom or scroll the text remains in a fixed position. I want the text to move along with the rectangles of the histogram when i scroll or zoom

[Uwe]

Of course you have to align your labels to the translated positions ( QwtScaleMap::transform ).

When you overload QwtPlotHistogram::draw instead you already have the maps - otherwise you get them using QwtPlot::canvasMap.

Uwe

[penny]

Could you point me to an example where this is done or QwtScaleMap has been used?

[Uwe]

Qwt ?

Uwe

[penny]

By example i mean some Qwt code where this has been implemented.. i cannot understand how to use it in my code..

[Uwe]

Plot items implemented in Qwt doesn't differ from those, that are implemented in application code.

Uwe

[penny]

Which application?

[d_stranz]

I'm overloading the drawCanvas method to display text on my QwtPlotHistogram. However when i zoom or scroll the text remains in a fixed position. I want the text to move along with the rectangles of the histogram when i scroll or zoom

What coordinates are you using for the text? Pixels or data coordinates? If you are using pixels, then of course the text will not move. So you need to define the text location in data coordinates, and use QwtScaleMap to transform the data coordinates into pixels when drawing.

Look at the Qwt source code for any QwtPlotItem class to see how to use the scale maps. QwtPlotCurve::drawSticks() (from Qwt 5.2) is a simple example:

00730 void QwtPlotCurve::drawSticks(QPainter *painter,
00731     const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
00732     int from, int to) const
00733 {
00734     int x0 = xMap.transform(d_data->reference);
00735     int y0 = yMap.transform(d_data->reference);
00736 
00737     for (int i = from; i <= to; i++)
00738     {
00739         const QwtDoublePoint sample = d_series->sample(i);
00740         const int xi = xMap.transform(sample.x());
00741         const int yi = yMap.transform(sample.y());
00742 
00743         if (orientation() == Qt::Horizontal)
00744             QwtPainter::drawLine(painter, x0, yi, xi, yi);
00745         else
00746             QwtPainter::drawLine(painter, xi, y0, xi, yi);
00747     }
00748 }

00730 void QwtPlotCurve::drawSticks(QPainter *painter,
00731     const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
00732     int from, int to) const
00733 {
00734     int x0 = xMap.transform(d_data->reference);
00735     int y0 = yMap.transform(d_data->reference);
00736 
00737     for (int i = from; i <= to; i++)
00738     {
00739         const QwtDoublePoint sample = d_series->sample(i);
00740         const int xi = xMap.transform(sample.x());
00741         const int yi = yMap.transform(sample.y());
00742 
00743         if (orientation() == Qt::Horizontal)
00744             QwtPainter::drawLine(painter, x0, yi, xi, yi);
00745         else
00746             QwtPainter::drawLine(painter, xi, y0, xi, yi);
00747     }
00748 }

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