plot an item in a list which keeps updating in Netlogo

I defined a set of attributes for some producer agents in the interface as global variables and then assigned them to a list (in the setup) which is also global and the agents will use its content.

The problem is that I cannot plot them.

Defined in the interface as global variables : p1-size = 9, p2-size = 13, p3-size = 14.5, p1-rate = 80, p2-rate = 50, p3-rate = 98

to setup

  ca  ; clear-all
  create-prod ; create producers
  set size-set ((list p1-size p2-size p3-size))
  set rate-set ((list p1-rate p2-rate p3-rate)) 

I want to plot and monitor how the rates changes, and I encounter an error by defining this :

plot item  0 size-set

Size-set is global, but the error says "ITEM expected to be a string or list but got the number 0 instead.

I'd appreciate any help regarding this.