Graphs of mail messages over time
I am looking for this also. This answer claims: "AppleScript doesn't offer any means of getting a list of replies to a message. My workaround is to get the message id of the message being replied to and search it in the sent messages."

TimeToReply.com also provides a similar service, but they have locked out new users pending product updates (unfortunately, they are moving from a cloud service to client plugin model).