i dont know if this is what you are after but goto excel spreadsheet and click on tools, then options, then on graph tab button, in there you should see option of plotting blanks as gaps or a zeros, select blanks, when i done this  and added a trendline it  worked fine.

Hi DaveHoughton,

I am using ExcelXP for a non-engineering spreadsheet. I am currently trying to lose weight to get back to my 1955 body weight, and have a chart showing all of my recorded weights with an encouraging trendline.

I am using a simple bar chart, with exponential trendline, assembled entirely via the chart wizard.  (A linear trendline looks just as encouraging).  My chart is scaled for 13 weeks, but my current data is only for 5 weeks.  The blank data ahead of me is not having any adverse effect on the trendline, which appears to take into account only the entered data.

So I seem to be doing exactly what you want, without any special cunning.  [My expertise with Excel is very primitive, and such 'cunning' would be beyond me].

In my data table, I have no data in any of the cells in which I will be entering future weight records.  If I enter zero in even one of them, I get a warning that some trendlines won't work with zero or negative values.  

Have you by any chance zeroed some/all of your future data cells, rather than simply deleting their contents?

Thanks for the suggestions. I am going to be away from my computer for the next week, so will be unable to confirm any results. However, your interest is greatly apreciated.
Regards,
Dave