#307 QwtPlotRescaler does not ensure the exact same scale on X and Y axes

[Sébastien Perseval]

Even if a QwtPlotRescaler has been installed and activated (using mode Fixed, Expanding or Fitting) the QwtScaleMap given to QwtPlotItem::draw() does not have exactly the same scale for X and Y.

This issue can be illustrated with the playground/rescaler example.

In RectItem::draw(), the call QRectF rect = QwtScaleMap::transform( xMap, yMap, d_rect ); does not return a square even if d_rect is a square.

If we resize the window of this example, we can observe that the error increases with an
increasing ratio for canvas.width() / canvas.height().

I propose the following version of RectItem::draw() to directly display the ratio in the plot item:

virtual void draw( QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap,
    const QRectF & ) const
{
    if ( d_rect.isValid() )
    {
      const double x_scale = qAbs((xMap.p2()-xMap.p1()) / (xMap.s2()-xMap.s1()));
      const double y_scale = qAbs((yMap.p2()-yMap.p1()) / (yMap.s2()-yMap.s1()));

      const QRectF rect = QwtScaleMap::transform(
            xMap, yMap, d_rect );

      const QString text = QString("%1 %2\n%3\n%4 %5")
                           .arg(d_rect.width()).arg(d_rect.height()).arg(y_scale / x_scale)
                           .arg(rect.width()).arg(rect.height());

        painter->setPen( d_pen );
        painter->setBrush( d_brush );

        if ( d_type == Ellipse )
            QwtPainter::drawEllipse( painter, rect );
        else
            QwtPainter::drawRect( painter, rect );

        QwtPainter::drawText(painter, rect, Qt::AlignCenter, text);
    }
}

In my own project, my PlotItem::draw() implementation relies on a ratio equal to 1 to correctly display a not distorted rotated rectangle.

[Anonymous]

For information: few more of my users have felt last week in this issue.
I fixed it at least for my own usage, by reimplementing the canvasResizeEvent() method to discard the margins, like this:

void MyPlotRescaler::canvasResizeEvent( QResizeEvent *event )
{
  rescale( event->oldSize(), event->size() );
}