How to put text labels on a figure

My program plots numerical data and makes figures. I´m currently using QPixmap to make figures. Now I´d like to label some very specific locations of the figure with text (preferably vertical text). How could I do that? I have a feeling that QPixmap may not be the best format for this. But if I wanted to use QPixmap, how could I do it?
Thanks!

You can use QPainter with QPixmap as Device

Yes, I know QPainter but how do I write and orient text with it?

Use QPainter::rotate and QPainter::translate and of course QPainter::drawText.