I have tried that technique as well and cannot seem to get it to work in my database either. My issue is that when there is a null value in the data, I want it to show a zero like the graph in that example starts with. I can't use discrete values on the axis because I want my trend lines to take the zeroes into account to make them accurate. Very frustrating, and if I find a solution I will be sure to post it here.
Is there null data or NO data in your data source for a particular year? If there is no data, then the information in the KB will not help. Tableau cannot create missing data values. For example, if you did not sell anything in 2009, Tableau does not create an entry for 2009 in your results. With Nulls, the row exists with some data, but some of the fields are blank. If this is the case, a lookup table can be created and joined to show a member for each year.
Hope this helps!