How To Recreate ZigZag Effect On Stock Data

I look at stock charts that allow an overlay of "zigzag patterns." This zigzag links price extremes with a line. The price extremes are defined according to "wave size" which is described as follows:

"Wave Size: The Most Important Setting... and the thing you will want to tinker with -- is Wave Size. Wave Size is a number that defines a high or low price extreme, by stating the number of surrounding bars on either side that must not be higher or lower than the candidate extreme. Another way to think of it is: Wave Size + 1 is the minimum distance between extremes of the same high or low type."

Here is an example of what a stock chart with these zigzags drawn in looks like. [Click on the image to zoom in and see it better.]

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The black diagonal line is built using a "wave size = 5" using the above definition. And the green one is, "wave size = 20"

I am trying to build this into a spreadsheet so I can play with the data. If I have four columns: (a:open)/(b:high)/(c:low)/(d:close) I cannot figure out how to build the columns after to do this zigzag. I understand what the line is, but it just doesn't make sense to me how to derive it from the data. how to get a column n that has the value of the zigzag (with wave size = 5 for example) I