cairo:rel-line-to (cr dx dy)

cr -- a cairo:context-t instance
dx -- a number with the x offset to the end of the new line
dy -- a number with the y offset to the end of the new line

Relative-coordinate version of the cairo:line-to function. Adds a line to the path from the current point to a point that is offset from the current point by (dx, dy) in user space. After this call the current point will be offset by (dx, dy).

Given a current point of (x, y),

(cairo:rel-line-to cr dx dy)  

is logically equivalent to

(cairo:line-to cr (+ x dx) (+ y dy))  

It is an error to call this function with no current point. Doing so will cause cr to shutdown with a :no-current-point status. The numbers of the arguments are coerced to double float values before being passed to the C function.