I built LoanSweetSpot.com to solve a personal frustration: standard mortgage calculators give you a grid of numbers, but I wanted to visualize the "knee" of the curve—the sweet spot where a small extra payment saves a disproportionate amount of interest (and time).
The Build Process: This was a pure "vibe coding" experiment. I acted as the product manager/architect, and Gemini acted as the junior dev handling the syntax.
Stack: Zero dependencies (except Chart.js via CDN), single index.html file.
Hosting: Cloudflare Pages (Free tier).
Time: From blank file to production in one sitting (~3-4 hours).
Features:
Interactive Payoff Curve: The graph is the controller. Drag the blue dot to adjust the term.
The "Danger Zones": Vertical red lines appear automatically when your total cost hits 2.0x or 3.0x the principal (predatory interest warning).
Privacy: 100% client-side math.
I’d love to hear what you think of the UX, or your thoughts on this workflow.