Those bell type curves show Percent CO2/HCO3/CO3 at any given pH. In fact
the equilibrium CO2 concentration is CONSTANT over a wide pH range.
Alkalinity is reported as mg CaCO3 but in fact is caused by the cations
in solution not paired with an anion such as CL- or PO4-3 etc. If you
change the ratio of anions to cations in solution, you will reduce the
alkalinity (such as by adding an acid).
However, adding an uncharged molecule such as CO2 does not affect the
alkalinity at all, just the pH.