Sorry for asking silly question but i am not able to get the anwer
Data look Like this i have 2 fields Date and Value and 3rd column shows the the required ouptput.
In straight table chart, define Date as dimension and Sum(Value) as expression. Check Full Accu,u;ation in the Accumulation box in the properties window Expressions tab to get cumulative (required) value.
If you are try this in script then use below script
If(RowNo() = 1, Sales, Peek('AccumulatedSales') + Sales) AS AccumulatedSales