MATLAB R2017a Legend displays wrong color when plotting multiple entries

When I execute the following code,

     f_0 = 1;
     x = [0:0.1:10];
     fhandle = @(x) 2*x;
     dx = 0.1;

     f = RK4(fhandle, x, f_0);
     freal = @(x) x.^2 + 1;
     feuler = ode_euler(x,f_0);

     figure
     plot(x,f,x,freal(x),x,feuler);
     legend('4th-order Runge-Kutta','Exact','Euler')
     xlabel("x")
     ylabel("f(x)")
     title("Runge-Kutta, Exact, Euler solutions to df/dx = 2x")

     function f = RK4(fhandle, x, f_0)
         %dx = 0.1;
         n = size(x);
         dx = (x(n)-x(1))/n;

         f(1) = f_0; 
         for i=1:n-1
             v1 = dx*fhandle(f(i));
             v2 = dx*fhandle(f(i)+dx/2);
             v3 = dx*fhandle(f(i)+dx/2);
             v4 = dx*fhandle(f(i)+dx);
             f(i+1) = f(i) +1/6*(v1+2*v2+2*v3+v4);
         end
     end

     function f = ode_euler(x,f_0)
         delta_x = x(2)-x(1);
         l_x=length(x);
         f=zeros(1,l_x);
         f(1)=f_0;
         for ii=1:(l_x-1)
             f(ii+1) = f(ii) + delta_x*2*x(ii);
         end
     end

Legend displays the wrong color lines for each plot. Assigning 3 separate plots doesn't help my situation. I have also tried assigning colors to the plots, as well as just about anything I can think of. When I assign colors to the individual plots, they each get the proper color but the legend displays each line as the color of the first graph.

How can I properly make the legend to display 3 different, correct, colors for my 3 plots on the figure?

Answer 1

[Itamar Cohen]

Hi Josh, I tackled the same bug, and finally fixed it by re-ordering the arguments to plot(); of course, you should change the arguments to legend() accordingly.

For instance, in your code, you may try to change your calls to plot() and legend() to:

plot(x,f,x,feuler,x,freal(x));

legend('4th-order Runge-Kutta','Euler' ,'Exact')

or to another permutation, until it works. Hope it helps.