Update plot using hold on inside a for loop

Question

I'm combining two plots using this code

plot(x1,y1,'.','MarkerSize',20,'Color','r');
hold on; grid on;
plot(x2,y2,'x','MarkerSize',10,'Color','b');

xlim([-a a]);
ylim([-a a]);

Now I want to change the values of x1,y1 and x2,y2 in order to have more than one point and one cross inside my figure. I tried to use a for loop where I compute new values, but every iteration this code generates another figure - whereas I want just one figure with all the points in it.

I did something like this:

for i=1:1:8
    % do things that compute x1,x2,y1,y2
    figure; hold on
    plot(x1,y1,'.','MarkerSize',20,'Color','r');
    hold on; grid on;
    plot(x2,y2,'x','MarkerSize',10,'Color','b');
    xlim([-a a]);ylim([-a a]);
    i=i+1;
end

I also tried to put the hold on just before i=i+1 but still give me a new figure.

Answer 1

There are several things you can do:

1. The simple solution would be to put the figure command outside the loop:

   figure(); hold on;
   for ...
     plot(x1, ...);
     plot(x2, ...);
   end
   

2. A better solution would be to first compute all values and then plot them:

   [x1,y1,x2,y2] = deal(NaN(8,1));
   for ind1 = 1:8
     % do some computations
     x1(ind1) = ...
     ...
     y2(ind1) = ...
   end
   figure(); plot(x1,y1,'.',x2,y2,'x');
   

3. The best solution (in my opinion) would be to update existing plot objects with new data points as they become available:

   [x1,y1,x2,y2] = deal(NaN(8,1));
   figure(); hP = plot(x1,y1,'.',x2,y2,'x');
   for ind1 = 1:8
     % do some computations
     hP(1).XData(ind1) = <newly-computed x1>
     hP(1).YData(ind1) = <newly-computed y1>
     hP(2).XData(ind1) = <newly-computed x2>
     hP(2).YData(ind1) = <newly-computed y2>
   end