Calculates the sample standard deviation (the square root of the sample variance) of all SourceChart's values over the time period.
Chart to calculate the maximum 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