stackR on x and y

[Fil]

closes #197

todo:

• this version returns channels r1 and r, which is not symmetric with stackX (x1 and x2). The dot mark knows only r for the moment, it has no notion of innerRadius/outerRadius.
• it would be nice to have an offset: r0 that would start the ring at innerRadius r0 (esp. for negative values)
• is it possible to rewrite (Xa, Xb) as a combined operation?