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Having that few values is definitely possible; it all depends on how often you're collecting data for that point, as well as how restrictive the exception and compression settings are. Essentially, compression will reduce the number of data points stored to the minimum required to still reflect the data within a certain tolerance. Thus, if you collected dozens of points that perfectly fit a linear trend, you would only need the two endpoints to be able to reproduce the values, instead of every data point. The image in this description of compression may help illustrate this.