For QPainter coordinate system, reflect over X axis and
        translate from center to top-left

def _convert(self, pos):
        """ For QPainter coordinate system, reflect over X axis and
        translate from center to top-left
        """
        px = pos[0] + self.logical_size.width() / 2
        py = self.logical_size.height() / 2 - pos[1]
        return px, py