You can see that near the equivalence point there is a very drastic change in the concentration of the free metal ion.
Calmagite indicator has two -OH groups with acidic protons.
The color of calmagite changes depending on whether or not these protons are present. Calcium and magnesium ions can displace the protons to form a calmagite-metal complex. The color of the complex is red. There is a limit to the amount of a solute that you can dissolve in a given solvent at a given temperature. If you add too much sugar to your iced tea, you'll see some undissolved sugar in the bottom of your glass.