If anyone else is interested I managed to get a workaround to produce the required effect using a stacked bar chart with three coloured bars:
B4X:
DemoChart.AddBar("Val", Colors.Green) '<-- Add coloured bars
DemoChart.AddBar("Val", Colors.Yellow) '<-- in this order
DemoChart.AddBar("Val", Colors.Red) '<-- for this to work
DemoChart.AddBarMultiplePoint("Value", ChooseColour(val))
I then have a sub named ChooseColour which sizes the three coloured bars depending on 'val':
B4X:
Sub ChooseColour(val As Double) As Double()
If val >= 75 And val < 85 Then
Return Array As Double(0,val,0) '<-- Only show the yellow bar
else if val >= 85 Then
Return Array As Double(0,0,val) '<-- Only show the red bar
Else
Return Array As Double(val,0,0) '<-- Only show the green bar
End If
End Sub