This diagram lays out the relationship between traditional Yogic chakras and the enneagram, as recounted in my book Chakras and the enneagram.
The majority of the work published at this location is based on this particular understanding. While there are other interpretations of the diagram, my own line of work begins with this one. All of the premises that are presented follow on this particular model.
Those interested in studying this particular question further may want to get a copy of the book.