The plot does not show anything because you are not actually plotting anything. You need to use indexing in the loop to store the values that are being calculated, otherwise they will simply get overwritten, and when you go to plot there is nothing to plot. Try this:
I_vec = 1:15;
T_iso = [400,600,800]; % isotherms
% preallocate output matrix:
out = NaN(numel(I_vec),numel(T_iso));
% loop over input vectors:
for ix = 1:numel(I_vec)
for tx = 1:numel(T_iso)
out(ix,tx) = XSteam('v_pT', I_vec(ix), T_iso(tx));
end
end
%
plot(out)
