I know the question is about Voltage level, but I would be wary about the current (amp) numbers on the chart above if also reading those. The graph pattern I would agree with, but the quoted current values should - IMO - be ignored as misleading.
Different manufacturers and different batteries have different limits but reckon on a maximum current of around 10% to 25% of
battery capacity as a rule of thumb, depending on your own
battery. In other words, if you had a lead acid
battery of 100Ah capacity, you would charge at a rate of 10-25A maximum (so if you want that on a per-cell basis, between around 1. 5A - 4A
(FWIW, on my Leoch AGMS, I use 14.4V as max voltage (goes higher when cold, so this time of year, if the charger has temperature compensation, don't be surprised to see higher), and a charge rate of 25% (so for my bank of 380Ah, my 120A charger is limited to 95A))