To do this, you need to feed rlocus a calculated value for sys. Let's say G(s) = 1/(2s+4+b) and you want to vary b. You need to form the closed-loop system, Gcl(s) = 1/(2s+5+b), and set the denominator equal to zero. Then take this equation, 2s+5+b=0, and solve it for -1/b, which gives -1/b = 1/(2s+6). Feed this into rlocus:
rlocus(tf(1, [2 6]))
Hope this helps you!
