When I was paying off my mortgage I took the floor plan from the home inspection and laid a grid over it, the calculated the per grid block price. With each payment and extra principal payment, I would fill in the blocks. I found it immensely satisfying to fill in rooms. I calculated the price of various rooms as well, so if I had some money saved up or got a bonus, I could drop a principal payment to pay off the bedroom, or whatever it may have been.

If brought to a web version, instead of my crude printed version, I could imagine a second grid of the same scale, that shows interest payments, both already paid and projected. I think visualizing how much more house one could have had if it didn’t all go to interest would be interesting. Having a “what if” option could show the paid off home filling in, while the interest floor space shrinks. Having a slider to dynamically perform that “what if” at different stages of the loan would be pretty striking I’d think. A lump some of principal today vs the same in 15 years, for example.