"Lloyd Nebres had a great idea, to take your weblog’s temperature by rating each day’s posts by whether you were hot or not, and then displaying a calendar view with colored dots that show at a glance how you were, or at least how you blogged, over the course of a month.
Phil Ulrich took the idea and ran with it, creating a PHP/Movable Type scheme for automatically generating average daily ratings that you display by the date header or in the calendar."
Here’s how Lloyd Nebres defined hotness: "…which of course begs the question: "significant" to whom, or about what? Well, significant to me. It’s a way for me to take each weblog day’s temperature, so to speak, and codify it in a very personal and subjective manner; there’s nothing scientific and objective about it at all."
Another idea would be to rate hotness depending on how many inbound links you had by day ("trackback hotness"?) and map this value to the calendar.