In reply to ACMan.

The blog post is from 10 years ago, but I believe I used PROC SGPLOT. I assume used the POLYGON statement. I probably used the COLORRESPONSE= option to get the gradient fill, and used the OUTLINE option for the boundary of the feasible region.

There is a similar example at https://blogs.sas.com/content/iml/2017/06/19/find-feasible-point-optimization-sas.html that includes the visualization code. Click on the last sentence in the article.