2.1.1577 Part 4 Section 18.104.22.168, serLines (Series Lines)
a. The standard states that this element specifies series lines for the chart.
In Office, if the parent is barChart, the series lines connect the top of adjacent data points within the same series; if the parent is ofPieChart, the series lines connect the secondary pie or column to the other slice.
b. The standard states that this element can be a child of any barChart element.
Office requires that the val attribute of sibling element grouping equal percentStacked or stacked when this element is specified.