Answer
Because [HCl] is such a strong acid, the y-axis corresponds to the [H+] because as you increase the concentration of hydrochloric acid, the concentration of [H+] should increase exponentially.
Work Step by Step
Because [HCl] is such a strong acid, the y-axis corresponds to the [H+] because as you increase the concentration of hydrochloric acid, the concentration of [H+] should increase exponentially.