It doesn't paint an arc like a pie chart.
It paints a circle partly occluded by a line from the edge of the circle.
Drawing an arc from 0 to 90 degrees should result in a quarter circle, with a right angle at the center.
Instead it results in tiny slice of the edge of a circle.
Sweeping the endAngle from 0 to 360 should fill it in like the hand of a clock sweeping from the center.
Instead it reveals a circle as if sweeping from the 3 o'clock position on the edge of the circle.