As per my understanding, you want to plot some points over X/D at 1, 2, 3, 4 on an existing plot.
To do that you can do something similar to this
% Define x/D vector
x_D = [1 2 3 4];
% Define matrix of u/U values
u_U = [1 2 3 4 ;
5 6 7 8;
9 10 11 12;
13 14 15 16];
% Plot u/U against x/D
hold on;
plot(x_D, u_U, 'x');
hold off;
To know more about 'hold', you can go through the following documentation :
