It shows the calculated pitch diameters of both the front- and rear-sprockets in 3 setup's. This can be calculated for both chain drive bikes as well as for belt drive bikes.
used to calculate the sprocket pitch diameter comes from the data
entered in the fields for the Final Drive (Front & Rear) and the pitch
of either the chain or belt in the 3 setup's on the main page.
What the graphs show is
a visualisation of the calculated dimensions of all 3 sprockets and
showing the calculated size either in inch or mm. As this is the
calculated pitch diameter, the actual sprocket outer
diameter will be different !
As explained on mentioned Help-page, there is a difference on how to interprete the shown diameter between chain and belt sprockets.
For chain sprockets, the actual outside sprocket diameter (Sd) will be slightly bigger as the calculated (Pd) one as the teeth are higher as the pitch diameter. However, the outer sprocket diameter is not important, what is important is the 'chain diameter' (Cd), that is the outside diameter of the chain when on the sprocket. It is the calculated pitch diameter (Pd) plus the height of a chain.
For belt sprockets the actual outside sprocket diameter (Sd) will be slightly smaller as the calculated pitch diameter as the teeth of the belt sprocket do not reach up to the pitch diameter. Also here this Sd is not important when checking if your sprocket will fit. What is important is the 'belt diameter' (Bd), that is the outside diameter of the belt when on the sprocket, which is the pitch diameter plus 2 times the height of the tensile cord to the top of the belt.
For most sprocket changes the difference in calculated pitch diameter between your current installed and the custom one considered, will indicate how much more space you need for your new sprocket as you will probably mount a chain or belt with the same height you currently have.
This will also help in determining if considering a smaller front sprocket, how much the chain will drop towards the swing arm so you can see if they will touch: half the difference between current and custom calculated Pd