The output by fsurf in the example given is being plotted for P in the range of 298 – 500 and T in the range of 10^(-10) – 10. This behaviour is observed because fsurf maps the values in the xyinterval argument to the symbolic variables in G as listed in:
symvar(G)
One simple workaround for this would be to replace [298 500 10^(-10) 10] with [10^(-10) 10 298 500].
Another approach would be to use symfun as follows:
syms T P
G= symfun(T*log(P),[T P]);
fsurf(G,[298 500 10^(-10) 10]);
For more details have a look at:
