Add description of resampling

content/plot-window/SummaryPlots.md

@@ -139,6 +139,12 @@ Right-click a composite plot in **Plots** and select **Split Into Single Curve P
 Right-clicking a plot in **Plots** window and selecting **Show Plot Data** will open a window containing the plot data as text columns. 
 The window displays plot data by day, week, month, quarter, half year and year.
 
+**Resampling Method Description**
+1. For aggregated values (**\*OPT** and similar), the resampled value is calculated based on linear resampling.
+2. For rate values (**\*OPR**), the total volume in the requested resample interval is computed. The rate value is then computed by
+
+      $Rate\_{period} = \frac{TotalVolumePeriod}{period}$
+
 The text content of this window is easy to copy and paste into Excel or other tools for further processing.
 It is also possible to save the text data to a file by the right-click command **Export to File**.