Thanks for explaining that. The plot makes much more sense now that I know that it's the derivative with respect to frequency. I will see if I can track down that thread you mentioned to learn more about this...There's a long and somewhat painful thread about group delay you can search for. Group delay is how quickly the phase angle changes with frequency (i.e. the derivative of phase wrt frequency). So a positive group delay means the phase is increasing wrt to frequency. A negative value means the phase is decreasing wrt to frequency.
If the plot of group delay is centered around some value (e.g. 0), it means the phase angle is oscillating around some constant.