Disc speed graph is not linear?

If I measure the disc speed (CAV), the rotation speed is constant.
However, the reading speed graph is inponential, not linear.
When looking carefully, that line has is slightly curved.

The rotation speed is really not constant at all in CAV.
It slightly decreases with regard to radius.

I know, that the speed decreases with CAV.
But the speed graph line is not a straight line, but slightly curved.
Can anybody explain?

Although it’s called CAV, the angular velocity is really not constant at all.
It slightly decreases when reading from the disc inner to outer.
If it were constant the graph would be a straight line

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Why does the disc drive not maintain speed?
(when accessing LBA 1 and then 300000 directly using IsoBuster sector view, the acoustic rotation sound did not pitch down).

I found out why:
The X axis is the data size and Y axis is speed.
If X axis was diameter, then the speed line would be linear.
But X axis is data length, which is higher per rotation at higher diameters, so the reading speed (data per time) keeps accelerating, flattening the line of linear velocity throughout the graph towards end.