The enabling disabling of the cache system is controlled by two defines (in jpg-config.php)
DEFINE( "USE_CACHE", true);
The first of these, USE_CACHE, is the master-switch which must be set to true to enable the caching system. The second switch READ_CACHE very seldom needs to be changed.
This second switch basically tells whether or not JpGraph should ever look in the cache. Setting this to false and the master-switch to true would then always generate an new updated image file in the cache and this new image would be send back to the browser. The main use for this (admittedly) strange setting is if you like to have the side effect of the script that a fresh image is always stored in the cache directory.
Once you have enabled the cache you must also make sure that a valid cache directory is setup. The cache directory is specified with the define
DEFINE( "CACHE_DIR", "/tmp/jpgraph_cache/");
You can of course change the default directory to whatever directory you fancy. But, you must remember one important thing. The cache directory must be writable for the user running Apache/PHP .