[metapost] Metapost and directions

[yves geunes]

Hi,

I am using metapost to create charts.
I noticed that

draw (-my_unit,0)--(my_unit,0);
ray[1]=(0,0) -- my_unit*2*dir 0.0000;
draw ray[1];

produces 2 lines on top of each other
but when I leave the origin and shift things:

draw (-my_unit,100)--(my_unit,100);
ray[1]=(0,100) -- my_unit*2*dir 0.0000;
draw ray[1];

the ray[1] is slightly angled.

Can I avoid this?

Thanks in advance,

Yves Geunes.