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For SeriesDataLabel s, the origin for the Y coordinate is the "Default" location for the Data label would in if DataLabelPlacement were set to 'Default'data label.
If you set Y 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.
The axis is such that larger Y values put the chart region lower in the chart.