Well, you already have the velocity data, so this shouldn't depend on the plot at all.
You can interpolate something like this:
% x and y values can't directly be used as subscripts
x = -50:50;
y = x;
v = peaks(101);
querypoint = [12.3 -17.7] % query point does not need to be integer-valued
vint = interp2(x,y,v,querypoint(1),querypoint(2),'bilinear')
