If you look carefully at the plots on the website, e.g.: http://plplot.sourceforge.net/examples-data/demo08/x08.03.png you'll see that the plots are indeed antialiased. For the website plots a driver based on the cairo library is used.
FORTRAN is not a flower but a weed -- it is hardy, occasionally blooms, and grows in every computer. -- A.J. Perlis