My Arduino software is an adapted version of DG7EAO software.

In it are two lines who enable some coorrection for correct spacing between various SWR lines.
    - REV=REV-1 enables to correct little differences between RFdiodes D1 and D2 (non-constant spacing between VSWR lines).
    - VSWR=VSWR*1000 can be used to correct the equal spacing between all SWR lines.
Check this at 1.8MHz with test resistors between 25 Ohms and 470 Ohms. (VSWR = "value test resistor" / 50).
Some other adaptations were made, f.i. waiting times and a level jump at start of a sweep.