centered text label

I have a QwtPlot where I want to place a label such that it is at a given coordinate on the y-axis but is always centered on the x-axis, even when the axis scale is updated. I'm really lame at this stuff, but I suspect that this would involve handling a "redraw()" method so that the x-position is recalculated. But I'm not sure.

If I want to draw the label at a gtiven coordinate in both x and y, I do the following:

Code:

---

text.setFont(QFont("Helvetica",24, QFont::Bold));
text.setColor(text_color);
QwtPlotMarker *label_mark;
label_mark = new QwtPlotMarker();
label_mark->setValue(x, y);
label_mark->setLabel(text);
label_mark->attach(thisPlot);

---

Simple. Is there something equally simple to have x instead be the center of the plot?

Okay, through trial and error I found a solution. I went into the source code for QwtPlotMarker and found the draw() method. I removed the section dealing with centered QwtText labels, changed the x coordinate from a mapped value to the center of the plot window, and bingo!

Code:

---

void draw(QPainter *painter,
	      const QwtScaleMap &, const QwtScaleMap &yMap,
	      const QRect &rect) const
    {
	if (parent->shade_power) {
	    int x = rect.width() / 2;
	    int y = yMap.transform(yvalue);
 
	    // 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);
	    fprintf(stderr, "draw called with x = %d; y = %d\n", x, y);
	}
    }
};

---

It works!

Another option is to use the regular API of QwtPlotMarker:

Code:

---

marker->setLineStyle(QwtPlotMarker::HLine);
marker->setPen(Qt::NoPen);
marker->setLabelAlignment(Qt::AlignCenter);

---

The Qwt docs are far from being good, but ...

Uwe

The documentation says:

The alignment determines where the label is drawn relative to the marker's position.

Parameters:
align Alignment. A combination of AlignTop, AlignBottom, AlignLeft, AlignRight, AlignCenter, AlgnHCenter, AlignVCenter.

See also:
labelAlignment()

Definition at line 312 of file qwt_plot_marker.cpp.

What I wanted was centered relative to the plot boundaries, not relative to the marker position.

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

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

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         }

---

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:

Code:

---

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);
	}
    }

---

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

Thanks!