When I export a CSV file from Garmin Connect, it doesn't include the Efficiency Index for each length. It does include the SWOLF for each length in the Avg SWOLF column, so I would also expect the Efficiency Index for each length in the Avg Efficiency column.
Both SWOLF and Efficiency Index for each length are included in the same graph under Details/Charts/Efficiency.
Please include the Efficiency Index for each length in the Avg Efficiency column in exported CSV files.