I have found this link on plotting variable in GrADS and yes, the manual is poorly written. I want to plot values equal to zero only.
Say I have anomaly from -10 to 10 and I want to extract and plot the contour of absolute zero only, how can I do this in GrADS?
reinit
open somefile.ctl
set gxout
d var #this plots the variable from -10 to 10