Calibration curve for single radial immunodiffusion
When the radial immunodiffusion method is used to measure antigen concentrations in samples, a plate or slide is set up using agarose containing an antibody or antiserum. Holes are punched out of the agarose to form wells into which antigen is dispensed. The antigen diffuses out into the agarose and when the antigen/antibody ratio is favourable an immunoprecipitate will form as a ring (immunoprecipitin ring). The final diameter of the ring is proportional to the initial concentration of antigen.

Addressing the question: isn't this simply explained because the values on the Y axis are squares of the diameters of the rings, and this includes the well into which the antigen is dispensed?