Market Shares per month

I'm rather new to Spotfire so may be asking obvious questions!

I'm trying to greate a simple chart with market shares for the selected products monthly.

The % of Total built in expression comes up with Sum([ScriptLineItems]) THEN [Value] / Sum([Value]) OVER (All([Axis.Color])) but this gives me the % share over the whole data set, and not the monthlies.

Previously in Qlikview I used the following expression: =Sum (ScriptLineItems)/Sum(total <ScriptMonth> ScriptLineItems) 



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