arc [ item <id> ] [at] <vector> radius <length> from <angle> to <angle> [ with { <option> } ]
Arcs (curves with constant radius of curvature, that is, segments of circles) may be drawn on multiplot canvases using the arc command. The at modifier specifies the coordinates of the center of curvature, from which all points on the arc are at the distance given following the radius modifier. The angles start and finish, measured clockwise from the vertical, control where the arc begins and ends. For example, the command
arc at 0,0 radius 2 from 90 to 270
would draw a semi-circle beneath the line x=0
2 cm