error in displaying graph

I'm solving the following code but the graph is not showing result of "D":

    N=10;
    D=zeros(1,N); %demand
    Q=zeros(1,N); %supply
    P=zeros(1,N); %Price
    d=1000; %constant of demand function
    q=100; %constant of supply function
    a=0.8; % sensitivity of demand to price
    b=0.5; %sensitivity of supply to price
    for t=2:N
      P(t)=(d-q-(b*P(t-1)))/a;
      D(t)=d-a*P(t);
      Q(t)=q+b*P(t-1); 
    end
    %plot
    Xvals=1:N;
  plot(Xvals,P,'b',Xvals,D,'r',Xvals,Q,'g')
    xlabel('time')
    legend('Price','Demand','Quantity')

the output is shown below:
however, when i change the *P* function only, the graph appears:

N=10;
  D=zeros(1,N); %demand
  Q=zeros(1,N); %supply
  P=zeros(1,N); %Price
  d=1000; %constant of demand function
  q=100; %constant of supply function
  a=0.8; % sensitivity of demand to price
  b=0.5; %sensitivity of supply to price
  for t=2:N
    P(t)=(P(t-1)-((d-q)/(a+b)))*((-b/a)^t)+((d-q)/(a+b));
    D(t)=d-a*P(t);
    Q(t)=q+b*P(t-1); 
  end
  %plot
  Xvals=1:N;
  plot(Xvals,P,'b',Xvals,D,'r',Xvals,Q,'g')
  xlabel('time')
  legend('Price','Demand','Quantity')

what can be the error? both results show some values of D

Answer 1

[Walter Roberson]

D and Q are identical and so draw on top of each other.

Answer 2

[Star Strider]

In the first code example:

for t=2:N
    P(t)=(d-q-(b*P(t-1)))/a;
    D(t)=d-a*P(t);
    Q(t)=q+b*P(t-1);
end

both ‘D’ and ‘Q’ have exactly the same values, so ‘Q’ overplots ‘D’.

In the second code example:

for t=2:N
    P(t)=(P(t-1)-((d-q)/(a+b)))*((-b/a)^t)+((d-q)/(a+b));
    D(t)=d-a*P(t);
    Q(t)=q+b*P(t-1);
end

they are not the same, so you see all three.

I leave it to you to determine any error that may be causing that.