Includes additional data in the visible range created by Offset/Limit.

For example:

<Plot DataLimit="6" DataExtend="2,1">

This will adjust the chart to display 6 items. It will however also include the the two items before the size shown, and the 1 after it. This will result in items being displayed beyond the edges of the chart, which can be employed for various visual effects.

One such effect would be a ClipToBounds="true" on a Curve. This would you to have the curve extend completely to the edges of the chart instead of ending at the visible data set.