Dynamic Plotting "Phantom" Line

Dear fellow MATLAB Users,

I have created a dynamic plotting system to be used with various x and y data points and have unwanted line appearing at the start of the plotting sequence. Here is my code as it currently stands:

 __________ CODE START __________

x=[0;6;2;8;4;10;6;12;8];
y=[0;2;4;6;8;10;12;14;16];

t=numel(x);
r=x(t);
nnn=y(t);

s1=subplot(1,1,1);
set(s1,'xlim',[1 numel(x)],'ylim',sort([0 r]),'nextplot','add')

p1=plot(r,nnn,'r-');
p2=plot(r,nnn,'g*');

axis([0 20 0 20])
set(gca,'ydir','rev')

for t=1:numel(x)
    r=x(t);
    nnn=y(t);

      set(p1,'xdata',[get(p1,'xdata') r],'ydata',[get(p1,'ydata') nnn])
      set(p2,'xdata',r,'ydata',nnn)

      pause(.5)

end

 __________ CODE END __________

Notice how there is a line extending from the origin, to the final data point. I cannot understand why this is occurring, nor what to do to remedy it. Any help that could be lent to solve this issue would be greatly appreciated.

Thank you,

~ Chris

[Jan]

Please use the code formatting as described at the link called "Markup" on this page.

[Chris Berry]

I apologize, I am not able to find the "Markup" link on this page, nor through a site search. Would you please provide me with a link so that I can better submit questions and answers? Thanks.

Answer 1

[Paulo Silva]

Initialize p1 with this code

p1=plot(nan,nan,'r-');

instead of

p1=plot(r,nnn,'r-');

[Chris Berry]

Exactly! Thank you, Paulo, for your help. It is people like you that make this forum great!

Answer 2

[Jan]

You get a connection from the origin to the last data point, because your program explicitely instructs that.

x = [0;6;2;8;4;10;6;12;8];
y = [0;2;4;6;8;10;12;14;16];
t = numel(x);
r = x(t);
nnn = y(t);
s1 = axes('xlim',[0, 20],'ylim',sort([0 20]), ...
          'nextplot','add', 'ydir','rev');

p1 = plot(r, nnn, 'r-'); 
p2 = plot(r, nnn, 'g*');

% Now [p1] is the last data point

for t = 1:numel(x)
  r = x(t);
  nnn = y(t);

    set(p1, 'xdata',[get(p1,'xdata') r], ...
            'ydata',[get(p1,'ydata') nnn])
    set(p2, 'xdata',r, 'ydata',nnn)
    pause(.5)
    % In the first iteration the origin is added to [p1]!!!
  end

I cannot guess, what you want to do instead. Perhaps you want to start your t-loop from 2 or remove the zeros from x and y.

[Jan]

Puh, the latency is cruel for me. I've typed the answer and waited 13 minutes until it appears completely in the web interface. Therefore I did not see Paulo's answer before.

[Chris Berry]

Ha. I still appreciate you trying to help me. Take care.

~ Chris