Evolution of Power and HP over time.
Saints are compared on the very same basis (stars, level, promote level, no starhill).
About updated saints (e.g. Sasha), they are obviously higher than they were originally. Using the axis, you can visually compare the quality (stats-wise) of their update.
Skills having an impact on the HP pool (% damage reduction, revive, etc.) are not interpreted. Therefore, a few Saints require way more HP to be taken down than their original HP bar.