You have two choices. The first choice is to create a new numeric data column with the row numbers in the data table. You can initialize a new column with a sequence of cardinal numbers or use a formula (simplly call the Row() function). The second choice is in Graph Builder itself. Right-click on a data column in the column list and select Row > Row. You now have a temporary (or virtual) data column to use in your plot. Use it just like a real column: drag it to the X drop zone. If you get the result you want and decide to keep the temporary column, right-click on it in the column list and select Add to Data Table.
