Hi Abhi,
have you tried to create a PNG file with one of the Dislin example programs (i.e. exa_c.c or exa_f90.f90)?
Do they show the same problem? I cannot reproduce it on my Ubuntu 20.04 LTS system with Dislin 11.5.
Are you using some colour routines in your program? You can also try to create a full colour PNG file
with the option IMGFMT ('RGB') before DISINI to check if their is any difference.
With best regards,
Helmut