How to have text centered inside each slice of a pie chart? - c++

Question

I would like to get the text labels (percentages) centered within each pie slice. It currently works a bit for two of the quadrants:
What am I doing wrong?
void PieChartWidget::paintEvent(QPaintEvent *) {
QPainter painter(this);
QRectF size;
painter.setPen(QPen(Qt::black, 2));
if (this->height() > this->width()) {
size = QRectF(5, 5, this->width() - 10, this->width() - 10);
} else {
size = QRectF(5, 5, this->height() - 5, this->height() - 10);
}
double sum = 0.0, startAng = 0.0;
double angle, endAng;
double percent;
for (int i = 0; i < qvValues.size(); i++) {
sum += qvValues[i];
}
for (int i = 0; i < qvValues.size(); i++) {
percent = qvValues[i] / sum;
angle = percent * 360.0;
endAng = startAng + angle;
painter.setBrush(qvColors[i]);
painter.drawPie(size, static_cast<int>(startAng * 16),
static_cast<int>(angle * 16));
startAng = endAng;
if (percent != 0) {
double draw_x = width() / 2 +
cos(PI * (endAng / 180.0 - angle / 360.0)) * this->width() / 4.0;
double draw_y = height() / 2 +
sin(PI * (endAng / 180.0 - angle / 360.0)) * this->width() / 4.0;
painter.drawText(draw_x, draw_y, QString::number(percent * 100) + "%");
}
}
}

Answer 1

On this line:
painter.drawText(this->width()/4,this->height(), QString::number(percent*100)+"%");
You seem to draw the percentage in the same place every time. You do successfully draw the percentage for each section, they're just being drawn in the same place every time. Try changing it to this:
painter.drawText(double(i + 1) * this->width()/4,this->height(), QString::number(percent*100)+"%");
And you'll see what I mean. By multiplying the x value by some changing value, the x position of each drawn text will change, and thus you will be able to see the different percentages being drawn.
If you want it to draw in each quadrant, then your code might look something like this:
# define PI 3.14159265358979323846
...
double draw_x = this->width / 2.0 + cos(PI * (end_angle / 180.0 - angle / 360.0)) * this->width / 4.0;
double draw_y = this->height / 2.0 - sin(PI * (end_angle / 180.0 - angle / 360.0)) * this->width / 4.0;
painter.drawText(draw_x, draw_y, QString::number(percent*100)+"%");
Basically, what's happening in the above code is I'm calculating the x and y coords of the middle of each slice. Then, I'm drawing the percentages in those positions.