I had the same need, so what I did was to add one extra point in my map with a reference velocity. Then I added some text close to this "reference arrow" to explain it. Of course, some empty space on the map is needed. Basically, it would be as follows:
[x,y] = meshgrid(0:0.2:2,0:0.2:2);
u = cos(x).*y;
v = sin(x).*y;
text(0.05,2.3,'Arrow scale: 1m/s')
Resulting in the following plot:
I hope it may help someone, as this post it quite old.