It's in the cartoon Phineas and Ferb.
Like the city of Springfield on The Simpsons, Danville is wherever the writers need it to be. There are several real world cities called Danville, but none of them fit the show's needs of being close enough to Mt. Rushmore to drive there in a few hours, yet have a huge port and beach. However Dr. Doofenshmirtz did point to it on a map of the US.
Although the map didn't even have state boundaries on it, let alone any finer detail, he points to an area in the mid-west to southwest of the country. However at times it has a major coastal harbor, and other times it sits on a river. However it is not a place that has a real latitude and longitude location and exists where it needs to for that episode because in the roller-coaster episode Danville is closer to the east coast.
Adding to the intentional ambiguity, Danville is situated in one of the three (anomalous) states that comprise the "Tri-State Area".
Some of the most fantastic places only exist within the pages of books, the frames of films, the panels of comics, even the lyrics of songs. You can't buy a plane ticket to any of these cities.
Some of these places are so thoroughly imagined by their creators, you forget they aren't real. Some leave gray spaces that only your imagination can fill in. Regardless, they all leave their mark on the individuals who wander their streets. Check out these 50 coolest fictional cities shared by Complex.com.