hum... pero si en la pagina dice claramente que tienen una version exclusiva para php5 (PHP5: v2.1.2 Released on the 26 May 2006) ademas en la parte de downloads te describen un poco mas sobre release :
Cita: PHP5: JpGraph 2.x - series
Note: The 2.x series is only for PHP5 (>= 5.1.x). It will not work on PHP4 Note that we only verify the library on PHP >= 5.1.x it might still work on versions < 5.1.x (i.e. 5.0) however we cannot gurantee anything for these old versions of PHP. In order to use the library on thses old versions of PHP you must manually change the MIN_PHPVERSION string to a version that Your installation supports. (If You are still using 5.0 You should really upgrade since 5.0.x have several known problems.)