learn.microsoft.com › en-us › officeChart trendline formula is inaccurate in Excel - Office Mar 31, 2022 · The trendline formula is used for an XY Scatter chart. This chart plots both the X axis and Y axis as values. Line, Column, and Bar charts plot only the Y axis as values. In these chart types , the X axis is plotted as only a linear series, regardless of what the labels actually are.