2.1.1538 Part 1 Section 184.108.40.206, splitType (Split Type)
a. The standard implies that the val attribute is optional.
Office requires this attribute.
b. The standard does not define the behavior when this element is omitted.
In Office, if this element is not specified the default behavior is to split by position with a split position value equal to the number of data points in the series divided by 3, rounded up.