E: Understanding this manual and API descriptions.
You will find the note "MANDATORY","MANDATORY [if not ... ]" and "OPTIONAL" notes at the beginning of each API. What they mean is
MANDATORY - this is a "must" API call. If you do not call this API your application will not work.
MANDATORY if not using yyy - if you are not using 'yyy', then this API is mandatory. E.g. some API's are not mandatory if you choose to use the default initialization.
OPTIONAL - this API provides "additional" functionality (e.g. drawing legends in a bar graph) to your charting, but it is optional and you have a choice to use it or not.
INTERNAL - used by one of the other functions, you will typically not need to use this function directly.
UTILITY - these API's are utility functions used by other API's. You will typically never use them unless you see a use in calling one for your own specific need.