Determines how an empty point is treated when the chart is drawn. If a value of 'Average' is used, then the chart is drawn ...

Determines how an empty point is treated when the chart is drawn. If a value of 'Average' is used, then the chart is drawn as if a point exists that has an average Y value, the average being calculated using the adjacent points.  If 'Zero' is used, then the chart is drawn as if a point exists with a value of zero.