Line with a fixed sized, according to starting point + angle

I need to implement a feature which shows an entities speed vector as a line whose origgin is from the entity’s center, length is fixed, and and direction is according to a given unit vector (it’s speed) (only in 2D mode).

I am having difficuly with implementing this feature.

When using polyline graphics, I have to supply coordinates (and not "origin, angle, length), so when I zoom in/out the size of the “vector” is not fixed.

I thought about using a billboard’s rotation feature, but unfortunately I am already using the billboard for something else…

Any help will be appreciated.
Thank you