How can I plot the Simpson's rule function?

I have been set the task to evaluate and plot the graphs representing the Rectangle Rule, Trapezium Rule and Simpson's Rule. I have managed to plot the first two, but the Simpson's Rule has me baffled.

My code begins with the calculations of each approximation and then goes on to plot what they represent.

I have attached the graphs that have so far been plotted...

Here is my Code:

%Default program
for run = menu('Run default program?','Yes','No');

      if run == 1;
          x0 = -10;
          x1 = 10;
          n = 20;

          h = (x1 - x0)/n;   %State value of h (strip width)
          x_val = x0:h:x1;   %Define x_val   

          y_func = x_val.^3; %Define y_func
          x_vala = x0:h/10:x1; %Define accurate x_vala
          y_fa = x_vala.^3; %Define accurate y_func

      elseif run == 2;

          x0 = input('please input x0: ');   %Input values of x0, x1 and n
          x1 = input('please input x1: ');
          n = input('please input n: ');  

          h = (x1 - x0)/n;   %State value of h 
          x_val = x0:h:x1;   %Define x_val  
          x_vala = x0:h/10:x1; %Define x_vala

          y_func = input('please input y_func as a function of x_val: ');   %Input value of y_func
          y_fa = input('please input y_func as a function of x_vala: ');  %Input value of y_fa
      end
  end

          total = 0; % Define totals  
          ttotal = 0; 

% Rectangle Rule calc

for r = 1:n;
    Ar = (h * y_func(r));
    total = total + Ar;
end

disp (['Rectangle Rule: ' num2str(total)]) %display total

% Trapezium Rule calc

for t = 1:n;
    At = ((h * (y_func(t) + y_func(t+1)))) / 2;
    ttotal = ttotal + At;
end

disp (['Trapezium Rule: ' num2str(ttotal)]) %display total

% Simpson's Rule calc

% Define Simpson even and odd sections
simpo = 0; %odd
simpe = 0; %even

for s = 2:2:n;
    simpe = simpe + (4 * y_func(s));
end

for s = 3:2:n-1;
    simpo = simpo + (2 * y_func(s));
end

totals = h/3 * (y_func(1) + simpo + simpe + y_func(n));

disp (['Simpson Rule: ' num2str(totals)]) %display total

%Plot Graph
subplot(1,3,1)
%rectangle rule graph and plot
for r = 1:n; 
    try
        hold on
        plot([x_val(r), x_val(r+1), x_val(r+1), x_val(r), x_val(r)], [0, 0, y_func(r), y_func(r), 0],'r-') %plot rectangle rule
    catch
        continue
    end
    hold off
end
hold on
plot(x_vala,y_fa,'k-','linewidth', 2) %plot accurate function
grid on
title('Rectangle Rule') 
xlabel('x_val') 
ylabel('y_func')
text(0,x1, num2str(total),'backgroundcolor', [1 1 1],'color', [0 0 0],'edgecolor', [0 0 0])
hold off

%Trapezium rule graph and plot
subplot(1,3,2)
hold on
plot(x_vala,y_fa,'k-','linewidth', 2) %plot accurate function
grid on
title('Trapezium Rule')
xlabel('x-val') 
ylabel('y-func')
text(0,x1, num2str(ttotal),'backgroundcolor', [1 1 1],'color', [0 0 0],'edgecolor', [0 0 0])
hold off
for t = 1:n; 
    try
        hold on
        plot([x_val(t), x_val(t+1), x_val(t+1), x_val(t), x_val(t)], [0, 0, y_func(t+1), y_func(t), 0],'b-') %plot trapezium rule
    catch
        continue
    end
    hold off
end

% simpson's rule graph
subplot(1,3,3)
hold on
plot(x_vala,y_fa,'k-','linewidth', 2) %plot accurate function
grid on
title('Simpsons Rule')
xlabel('x-val') 
ylabel('y-func')
text(0,x1, num2str(ttotal),'backgroundcolor', [1 1 1],'color', [0 0 0],'edgecolor', [0 0 0])
hold off