What specific expression is MATLAB Coder highlighting? What types do the variables in your code snippet have? What ranges are known? Did you turn dynamic memory allocation off?
At a guess, you have turned dynamic memory allocation off and the problem is this expression:
Proving that this has a bounded size requires symbolic reasoning beyond MATLAB Coder's abilities. If you rewrite it as follow it should work provided nt is bounded
However, without reproduction steps this is just a guess.
Note, code generation does not generate code for these plotting routines. They will be evaluated using the MATLAB execution engine when run in MATLAB and not show in standalone code at all.