2.1.1499 Part 1 Section 18.104.22.168, period (Period)
a. The standard implies that the val attribute is optional.
Office requires this attribute.
b. The standard states that this element is ignored for non-moving average trendlines.
Office requires this element to be included when the val attribute of trendlineType equals movingAvg and not otherwise.
c. The standard states that the val attribute has a maximum value of less than or equal to 255.
Office requires that the upper bound of @val is the number of categories on the series minus 1. If the @val is greater than the number of categories minus 1 but less than 256 the file loads and saves correctly but the trendline will not be displayed