The question title says it all.
I am looking for a geometric proof for the fact that the group law defined on elliptic curves is associative.
I've heard somewhere about something on the internet about 2 cubics intersecting at 8 points must have a ninth point in common but I've never understood what that meant. Maybe this is what I'm looking for.
If a well-known reference has this result and I could easily find it in a library (or better yet over the internet, which I did not manage to find), then I'd accept this as an answer too.
Thanks in advance,