Paul (Adam?) - I think all that you are missing is in your assignment
y_values(:,i1)= y_values(i1) + dt * myode(t_values(i1),y_values(i1))
Remember that y_values is a 2D array..so you will need to access both rows for every column. The assignment does this with
y_values(:,i1)=
and so you will need to do the same (else you will get assignment errors or maybe even errors with myode since it might be expecting a 2x1 array as the second input to this function.
Secondly, each step of the algorithm uses the results of the previous iteration. Currently, your code is using i1 on both sides of the assignment. Since the minimum value of i1 is 2, then you can access the previous value with i1-1. Try the following
y_values(:,i1)= y_values(:,i1-1) + dt * myode(t_values(i1-1),y_values(i1-1));
