If we have in mind the following procedure:
BACo3(s) + H +1 -> Ba +2 + HCO3 -1
CO3 - 2 + H+1 -> HCO3 -1
As nitric acid is added then {CO3 -2 } decreases meaning that more BaCo3 can dissolve. The graph you need to use is the one that the curve goes down and closer to solubility and goes up on conc.
