Canvas redraws only on resize

[rarara]

Hi,
I need help with a js animation on a canvas in my qml app.
I want to draw a circle that starts at full circumference but as the variable progress goes down, the circles circumference gets smaller as well. So I have the basic code working, it draws the circle correctly I think. But:
it only redraws when I resize the window! Drives me nuts.

pastebin

In the code you can see several parts where I commented out some function calls which are supposed to clear or redraw the area but didn't for me.

I (and my future cardiologist) would be really grateful if you could help!

If it helps, I am on Ubuntu 15.04, Qt 5.5 and tried as ubuntu app as well as pure qml, same results.