density ht, pressure ht and landing charts.

Air density is criticial to aircraft performance, which is why it
surprises me that both Piper and Cessna Landing performance charts
show the climbout weight limit as a function of pressure height and
not density height. As I understand it ( at least from Australian
Regs) the aircraft needs to have a minimum climb gradient of 3.2% in
the event of a go around being required. Surely the achievable climb
out gradient must be density ht dependant. I dont think I have a
problem with the the understanding of what pressure and density hts
are and how to determine them, but I cant reconcile my understanding
of what they are with the performance chart. .Am I missing something
here?
Terry