First attempt to resolve a bug with horizontal pie offset.

It appears that the offset.left config is always ignored, as the maxRadius is being used to check for overflow instead of the actual pie width/radius.

My fix calculates the width as the actual plot using a function, getRadius(), and replaces other occurrences of the same code to prevent repetition. 

Reference SO quesiton:
http://stackoverflow.com/q/18805888/268093