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For SeriesDataLabel s on a column or bar chart X is the offset perpendicular to the bar or column from the default position in 1/4000 of the ChartArea. For SeriesDataLabel s on a pie chart X is offset angle from the default position in radians. For SeriesDataLabel s in all other charts types X is the offset from the default x position in 1/4000 of the ChartArea, the origin for the X coordinate is the location the Data label would in if DataLabelPlacement were set to 'Default'.
If you set X to a value that would put the Chart region off the chart, then when the workbook writes, the value will be clamped such that the region will still appear on the chart.