We have a built in function that works perfectly for this: CumulativeSum.
In my example, I want to create a monthly running total of the [Cost] column in chronological order to plot in a line chart.
I sorted the [Month of Date] column ascending and grouped by the [Month of Date] column. I then created an aggregate calculation to Sum() the [Cost] column so I could get the [Monthly Cost]. Finally, I created an aggregate calculation to CumulativeSum() the [Monthly Cost] column, outputting the running total.
I could then use this [CumulativeSum of Monthly Cost] column as the Y-axis for my line chart.
Important note: cumulative functions depend on the order of the given column. So if you change the sorting of that column, you could change the result!