Really cool and fast visualization! Upvoted

This might help the OP (also other readers and for reference) - It might be competing with the OPs app, but Here.com has a solution for this in one of their developer APIs(https://developer.here.com/documentation/routing/topics/requ...), it returns an isoline(contour) based on multiple options(max distance, max time or max fuel consumption) similar to the OPs solution