Update plot using hold on inside a for loop - matlab

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:
The simple solution would be to put the figure command outside the loop:
figure(); hold on;
for ...
plot(x1, ...);
plot(x2, ...);
end
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');
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