2.1.1432 Part 1 Section 21.2.2.9, axId (Axis ID)
a. The standard states that values of the val attribute are defined by the XML Schema unsignedInt datatype.
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Office restricts the value of this attribute to be at most 2147483647.
b. The standard does not place any restrictions on the val attribute of the axId element.
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Office requires that the val attribute of this element matches the val attribute of an axId element on a catAx, dateAx, serAx, or valAx element on the chart.