2.1.1506 Part 1 Section 21.2.2.135, 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

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