What do you want to filter on your graph ?
This method : SelectValuesInColumn allow you to filter in the dimension of your extension.
For example you have a graph with : Client as dimension and Sum(Sales) as Expression.
With SelectValuesInColumn you can only filter Client (because it's the only dimension inside your extension).
After you need to get all your value at once and call SelectValuesInColumn with your parameter. All the value must be in an array when you call SelectValuesInColumn. I usually use SelectTextInColumn
Hope this help.