2.1.1463 Part 4 Section 188.8.131.52, crossesAt (Crossing Value)
a. The standard states the value's data type is determined by the parent element.
In Office, the value data type is inferred from the type of axis being crossed, as follows:
When the axis to cross is specified by a valAx element, the value is a decimal number on the value axis.
When the axis to cross is specified by a dateAx element, the date is defined as a integer number of days since Dec. 31, 1899, unless date1904 is set, in which case they are defined as an integer number of days since Dec. 31, 1903.
When the axis to cross is specified by a catAx element, the value is an integer category number, starting with 1 as the first category.
b. The standard states that the possible values for the val attribute are defined by the XML Schema double datatype.
In Office, INF, -INF, and NaN are not supported values.