When intellisense, auto-completion, or the compilation needs a reference that it knows about it will not tell you what it is.
add package reference to FSharp.Charting
FSharp.Charting.Chart.Line [ 1,2;3,4] |> fun x -> x
then hit . after the x
tab adds `<note>` to the code, and the note won't show any tooltip about what reference is needed.
screen shot (dimmed because ctrl dims autocomplete, and is needed to tell windows to capture pop-ups in screenshots)
or (both should show up) at Screen shot