centered text label

[Uwe]

A marker might represent a point or a coordinate only ( HLine, VLine ). In the second case the text is aligned to the coordinate + the canvas rectangle, what is exactly the requirement you have.

F.e the bode example shows such markers.

Uwe

[djconnel]

Ahh! Sorry. You're right, of course. From the source code:

00204         if (d_data->style == HLine)
00205         {
00206             if (d_data->align & (int) Qt::AlignLeft)
00207                 dx = r.left() + xLabelDist - tr.x();
00208             else if (d_data->align & (int) Qt::AlignRight)
00209                 dx = r.right() - xLabelDist + tr.x();
00210             else
00211                 dx = r.left() + r.width() / 2;
00212         }

00204         if (d_data->style == HLine)
00205         {
00206             if (d_data->align & (int) Qt::AlignLeft)
00207                 dx = r.left() + xLabelDist - tr.x();
00208             else if (d_data->align & (int) Qt::AlignRight)
00209                 dx = r.right() - xLabelDist + tr.x();
00210             else
00211                 dx = r.left() + r.width() / 2;
00212         }

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Which is basically what I had (except I assumed r.left() = 0).

The exercise hasn't been a waste, however. In this example, I align the text to a diagonal of a plot, where for each label, x * y = watts:

void draw(QPainter *painter,
	      const QwtScaleMap &xMap, const QwtScaleMap &yMap,
	      const QRect &rect) const
    {
	if ((rect.width() > 0) &&
	    (rect.height() > 0)
	    ) {
	    // draw the label along a plot diagonal:
	    // 1. x*y = watts * dx/dv * dy/df
	    // 2. x/width = y/height
	    // =>
	    // 1. x^2 = width/height * watts
	    // 2. y^2 = height/width * watts
 
	    double xscale = fabs(xMap.transform(3) - xMap.transform(0)) / 3;
	    double yscale = fabs(yMap.transform(600) - yMap.transform(0)) / 600;
	    double w = watts * xscale * yscale;
	    int x = xMap.transform(sqrt(w * rect.width() / rect.height()) / xscale);
	    int y = yMap.transform(sqrt(w * rect.height() / rect.width()) / yscale);
 
	    // the following code based on source for QwtPlotMarker::draw()
	    QRect tr(QPoint(0, 0), text.textSize(painter->font()));
	    tr.moveCenter(QPoint(x, y));
	    text.draw(painter, tr);
	}
    }

void draw(QPainter *painter,
	      const QwtScaleMap &xMap, const QwtScaleMap &yMap,
	      const QRect &rect) const
    {
	if ((rect.width() > 0) &&
	    (rect.height() > 0)
	    ) {
	    // draw the label along a plot diagonal:
	    // 1. x*y = watts * dx/dv * dy/df
	    // 2. x/width = y/height
	    // =>
	    // 1. x^2 = width/height * watts
	    // 2. y^2 = height/width * watts

	    double xscale = fabs(xMap.transform(3) - xMap.transform(0)) / 3;
	    double yscale = fabs(yMap.transform(600) - yMap.transform(0)) / 600;
	    double w = watts * xscale * yscale;
	    int x = xMap.transform(sqrt(w * rect.width() / rect.height()) / xscale);
	    int y = yMap.transform(sqrt(w * rect.height() / rect.width()) / yscale);

	    // the following code based on source for QwtPlotMarker::draw()
	    QRect tr(QPoint(0, 0), text.textSize(painter->font()));
	    tr.moveCenter(QPoint(x, y));
	    text.draw(painter, tr);
	}
    }

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I think this would be more challenging within the standard API.

Thanks!