I haven't reproduced it yet, so it's rather not to forget to test it.

Take this C# code:

await Microsoft.FSharp.Control.FSharpAsync.StartAsTask(GenericChart.fromFigure(fig).ToBase64PNGStringAsync())l

There's a suspection that it either deadlocks or some other issues on borders between F#'s async and C#'s tasks.

Edit:

Chart.Line([1., 1.; 2., 3.; 3., 3])
|> Chart.toBase64PNGStringAsync()
|> Async.RunSynchronously
|> printf "%s"

never terminates