28.4527 setxunit %% 10 mm
%60 setxunit
-3 7 setxrange
-1 3 setyrange

%[1 1 .5 .5 .1 .1] {orange} Quadrillage

traceaxes
axesarrow
unites

/O {0 0} def
/A {1 0} def
/B {3 1} def
/cer {O 1} def
/M {18.5 cer cpoint} def

O B droite
A O B 2 (<-) tripointarcarrow

1.5 setlinewidth
O M (->) line