This relathionship doesn't have anything to do with titration curve - it describes dissociation of acid and conjugated base. For example for acetic acid dissociation constant:
COOH <-> H+
Ka = [H+
conjugated base is acetate, and its basic dissociation looks like:
O <-> CH3
COOH + OH-
Kb = [CH3
(water concentration assumed constant and omitted).
You may check that Ka*Kb = Kw, thus pKa + pKb = pKw (it is also described in the lecture on Bronsted-Lowry theory
So this is a general relationship - it holds for titration curve as well, but that's only a specific case, one of many.