Choose New Data Set from the Data menu, to add a new data set with matching columns to your table. If the page doesn't already have a table, one will be created with X and Y columns.
Note: If New Data Sets Have (X,Y)" in Preferences is checked, then new data sets will have just two columns: X and Y. If the option is unchecked, new data sets will be copies of the first data set.
You can edit the column options by either double-clicking the column name in the data table or choosing Column Options from the Data menu.