Here's what's happening. MATLAB is interpretting your expressions as necessary, when you say the limits are -1:0.5, it runs this:
-1:0.5
ans =
-1 0
And because the default stride is 1, it returns -1 and 0. These are then interpretted as your limits. I think you probably meant
ezplot(F,[-1 0.5]) or similar
Here's an example:
syms t
F = sin(t)*cos(t);
ezplot(F,2*[-pi pi])
