Add a WindowFunctionGuide

visidata/guides/WindowFunctionGuide.md

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+# Perform operations on groups of rows
+
+The window function creates a new column where each row contains of rows before and/or after the current row in the source column.
+
+Window functions enable computations that relate the current window to surrounding rows, for example:
+- cumulative sum
+- rolling averages
+- lead/lag computations
+
+## Window functions operation on columns
+
+Create a window for a column. The new column will contain the current row, and also any before or after rows specified when creating the window.
+
+- {help.command.addcol-window}
+
+To conserve memory and speed with large windows, one approach is to:
+1. add any expressions that operate on the window expression.
+2. Freeze the sheet [:keys]g'[/].
+
+## Examples
+
+After creating a window, use a python expression to operate on it.
+
+For example, given a windown column 'win', to create a moving average of the
+values in the window, add a new column with a python expression.
+
+```
+=sum(win)/len(win)
+```
+
+### Create a cumulative sum 
+
+- set the before window size to >= the total number of rows in the table, and the after rows to 0.
+- add an expression of `sum(windows)` where `window` is the name of the window function column.
+
+### Compute rank 
+
+https://github.com/saulpw/visidata/discussions/2280#discussioncomment-8314593
+
+### Compute the change between rows
+
+1. Create a window function of size 1 before and 0 after
+2. Add a python expression. Assume the window function column is 'win', and the current (integer) column is named seconds:
+    `=win[1] - win[0] if len(win) > 1 else None`
+
+