'SaveChartAs' is not defined

On linux (Ubuntu 15.04, vivid with Xamarin Mono), the code below, run with "fsharpi test.fsx" gives the error:

test.fsx(27,4): error FS0039: The field, constructor or member 'SaveChartAs' is not defined

Thank you very much for your help!

Note: ch.ShowChart() displays a window with the chart as expected. For this, I run the code by first executing

fsharpi

and then
# load "test.fsx";;

(Otherwise the chart windows is closed right after creation).
# I "packages/FSharp.Charting.Gtk.0.90.12/lib/net40"
# I "packages/FSharp.Charting.Gtk.0.90.12"
# load "FSharp.Charting.Gtk.fsx"

open FSharp.Charting
open FSharp.Charting.ChartTypes

let ch = Chart.Line [ for x in 1.0 .. 100.0 -> (x, x *\* 2.0) ]
ch.ShowChart()
ch.SaveChartAs("test.png" ChartTypes.ChartImageFormat.Png)