You can set the radius and the variant.donut.width properties:
obj.Set('radius', 250)
obj.Set('variant', 'donut')
obj.Set('variant.donut.width', 50)
And there's a demo (called demos/donut-in-donut.html) that demonstrates the radius setting here in the RGraph download archive which you can get here:
https://www.rgraph.net/download.html#stable
To write inside the ring you can use the drawing API text object. The X/Y coords are:
obj.ondraw = function (obj)
{
var x = obj.centerx
var y = obj.centery
}