Amir Salihefendic wrote a fantastic post explaining the nitty-gritty of how Reddit’s hotness algorithm works, so I won't bother repeating that here. Instead, I'll jump right into the visualization showing us Reddit's window of virality.
The y-axis indicates the post's current score; the higher up, the higher the post's score. The x-axis indicates the post's current age; the more to the right, the older the post is. The color indicates the post's "hotness" or virality, with darker shades of red for viral posts and darker shades of blue for posts that don’t stand a chance of getting to Reddit's front page.