Calculate Cumulative Graph

[#1]

I am struggling with a cumulative expenditure chart (see below).

Essentially the chart consists of:

Dimension1: [Date] (continuous X axis)
Dimension2: DimensionX
Expression1: sum(AmountGBP)

The chart is set to full aggregation however I cannot get it to calculate values beyond the date of last expenditure for each dimnsion. Beyond this point the series disappears, when I want it to continue to the same value.

Any suggestions? I have played around with rangesum and other functions but cannot get it to work.

Many Thanks.