resolve PHP 5.3 deprecations in jpgraph

http://techblog.wimgodden.be/2010/06/24/automated-php-5-3-compatibility-testing-for-your-old-code/

$ git clone git://github.com/wimg/PHP53Compat_CodeSniffer.git PHP53Compatibility
$ phpcs --standard=PHP53Compatibility . -v
fixing return value deprecation

[PHP 5.3] Assigning the return value of new by reference is deprecated in PHP 5.3
fix jpgraph deprecations