Specify this variable when the
ID Variable values each cover multiple rows.
The Row Index Variable indexes rows within values of the
ID Variable, and its values must be unique within each level of the ID Variable. For example, if you are working with data that have multiple replicate
observations per gene, you would specify the column that contains a variable indexing the replicates.
If you specify only one variable in the Plotting Variables field, then the
Row Index Variable is used as the second plotting variable.