Google flights can be difficult, but if you come back to the explore map from an F9 search, then you get the map with just F9 showing.

So, go to google flights and put in something like DEN to LAS, and choose only Frontier. Then change the LAS to United States and hit return. The Explore map should come up for the whole country but with only F9 flights. Then you can change the origin city to what ever you want as well as the dates. Then save that to your desktop to get it back immediately when you want just this search.

I would put my link here but it is hundreds of alphanumberics long and I am afraid that some of my cookie info is embedded in it.