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:
CH
3COOH <-> H
+ + CH
3COO
-Ka = [H
+][CH
3COO
-]/[CH
3COOH]
conjugated base is acetate, and its basic dissociation looks like:
CH
3COO
- + H
2O <-> CH
3COOH + OH
-Kb = [CH
3COOH][OH
-]/[CH
3COO
-]
(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.