You say that you have a problem with "one" of your lines, but I think you have a deeper conceptual problem with how you tried to do this.
Look at the result of just this code:
a=[-1,-1];
b=[-1,1];
figure
plot(b,a,'-ok')
axis([-2,2,-2,2])
I assume you are intending your point "a" to be the bottom left, and point "b" to be the upper right. But the syntax you used to make the plot is not accurate in that case. When you do
plot(x,y)
you need to make sure that it is that all x coordinates are the first input, and all the y coordinates are the second input. So the line you want would be plotted like this:
a=[-1,-1];
b=[-1,1];
figure
plot([b(1) a(1)],[b(2) a(2)],'-ok')
axis([-2,2,-2,2])
I would do your whole rectangle like this:
x = [-1 -1 1 1 -1]; % All of the x coordinates
y = [-1 1 1 -1 -1]; % All of the y coordinates
figure
plot(x,y,'-ok')
axis([-2,2,-2,2])
