I am plotting a chart showing monthly index yields.

My first date is 29th May 1981 and the last 30th June 2009. My x-axis
currently has two yearly spaced dates showing (ie 29th May 1981, 83, 85
etc...) but I really want it to be labelled 29th May 1981 then 31st December
1982, 31st December 1984 and so on...to 31st December 2008.

How can I achieve this? Am happy to add the first one manually but I really
want all of the December ones to be the general default.

Thanks

Rachel