Trendline in Stacked Column Chart

I have a stacked chart which shows data for 6 separate series identified by
month for 2010. What I am trying to do is include the data for each month
but only project a trendline using the data for months that have already
elapsed (for instance I would only want the trendline at the end of this
month to show the trend based on data for Jan, Feb, Mar and Apr).

I cant get a trendline on the graph at all let alone one that is constrained
to specific data.

I have tried including an additional data series which is the total of the
other 6 and then plot this on a secondary vertical axis in the hopes that I
could use this to create a trendline.

Can anyone help