The canvas arc is basic and very low level- see
http://wiki.flightgear.org/Canvas_Nasal_API#arcSmallCCWTo1. All co-ordinates are cartesian, not polar. (not a good idea for drawing arcs - but that is all we have.IMHO Centre co-ordinates, radius, start angle , finish angle would be much better)
2. End co-ordinates are relative to the start position of the arc.( also bad, why not have the same co-ordinate system for both start and stop ?)
3. Any one of 4 routines may be used, depending on the direction and size of the arc that you are trying to generate.
4. arcSmallCW and arcLargeCW draw clockwise arcs from the start position to the finish position.
5. arcSmallCCW and arc LargeCCW draw counterclockwise arcs from the start position to the finish position..
6. "Small" and "Large" are used to say that you want a large ( <180deg) or small (>180deg) arc.
Hope this helps.
Alan