BusyMail is a Python program that provides statistics about the user's email usage. It reads a given folder on an IMAP server (for me, the "starred" messages in GMail) and records which messages are present. See the GitHub page for more information.
Andrea's vital statistics
For me, the messages that are "starred" are a measure of how busy I am: when I have to do something connected to the message, I star it. (After all, "email is a to-do list written by others".)
More in detail, the number of messages is proportional to the current "stress", while the age of the messages is propertional to the current "procrastination".
The following are the complete historical plots.
Complete stress plot (number of starred messages):
Complete procrastination plot (mean message age):
Complete procrastination plot (median message age):