Agreed, the UI is really nice and straightforward to use! My only suggestion would be to highlight the corresponding graph point when hovering over an episode in the list, and vice versa.
Did you use Highcharts because you were used to it or because you weren’t able to find something with enough functionality in Elm? Have you taken a look at elm-plot? Having everything in pure Elm would almost certainly make my highlighting request much easier =)
As for the pendingRequests field in your model, have you looked at an approach like remotedata?