You can find a list of all the power types here:
http://wow.go-hero.net/framexml/16016/Constants.lua#119
Chi currently uses SPELL_POWER_LIGHT_FORCE. There is a corresponding SPELL_POWER_DARK_FORCE but it's not used for anything as far as I know.
Don't forget to register for the UNIT_POWER event in your panel's OnLoad, too.