Suggestions:
In his case, you are actually plotting two root locus plots on the axes. One is the original of the system, the other of the negative of that system. The plot will end up looking like it is plotting both positive and negative gains for the original system, but actually, they are two separate systems and all gains plots are will still be positive for both systems.
If you want positive and negative gains for a system, you can pass in a specific vector of gains as the 2nd argument to 'rlocus'.
For example:
h = tf([2 5 1],[1 2 3]);
rlocus(h,-10:.0001:10)
Also, please note that for certain ranges of gains, you will need a really fine resolution to make the curve look smooth.
Please refer to the below documentation link which provides more information about 'rlocus' https://www.mathworks.com/help/control/ref/rlocus.html
