Calculates the sum of all values from IndexLow to IndexHigh in the SourceChart and returns the sum
Chart to calculate the sum from
Lower bounds index into the SourceChart
Upper bounds index into the SourceChart
TimeStamp of the new value
Note that this function is a class procedure so it may be called even if the TRSCustomChartFunction is not instantiated.
The function can also be called by custom functions. For example, you could create a custom function that calls this function and then does something with the result