SSG, I would consider a Visio diagram to be the VERY VERY MINIMUM evidence you should supply. I have an aunt in Canada, and my lawyer advised me to get the following:
1 Notarized letter from my Dad stating he is my aunt's brother.
2 Notarized letter from my grandmother stating she is my dad and aunt's mother.
3 Notarized letter of support from my aunt.
4 Copy of her landing papers.
5 My aunt's birth cert and my dad's birth cert, showing they have the same parents and place of birth. Then, my birth cert shows my relationship with my dad.
6 A "visio" (or MS Word) tree is then only a diagram to aid them evaluate the above.
I'm european, so I don't know if your home country's birth cert ahs parents names, but provide A LOT of info - especially a notarized letter of support from your uncle.