I tried your suggestions. What I did was:
    Added "$image->interlaced(undef);" in the line after I call the plot method.
    Added "$hPdf->save;" in the line after I called the stringify method.
    Changed the dimensions of the mediabox to be 1200x900.

Unfortunately, after all this I am still having the same problem. When the PDF file is created it starts printing out "substr outside of string at /usr/lib/perl5/site_perl/5.8.3/PDF/API2/PDF/ImagePNG.pm line 137.".

Do you know what versions of these packages you are running? Maybe there is some incompatability if I am on the bleeding edge of releases.

Thanks for your help,
John