Max -

A trendline can only be made with all points in a series. As an alternative to
John's suggestion, make a series with only the last 13 data points:

Observation Date Data Partial
1 1/1/2005 56
2 1/2/2005 67
3 1/3/2005 65
4 1/4/2005 56
5 1/5/2005 67
6 1/6/2005 68
7 1/7/2005 75 75
8 1/8/2005 67 67
9 1/9/2005 69 69
10 1/10/2005 85 85

Plot both series, the full set and the partial set, and make your trendline using
the shorter series. If you want, you could hide these points by formatting the
series to use no marker. The trendline will span the entire range of data, even
though it is based only on the plotted points.

To get a trendline that only spans the appropriate points, make your second series
as an XY Scatter series using only the points in the Partial column and the
corresponding dates. If you started with a line chart, select just the short series,
and choose Chart Type from the Chart menu to change its type; then choose Chart
Options from the Chart menu, and uncheck the secondary axis boxes on the Axes tab.

- Jon
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Jon Peltier, Microsoft Excel MVP
Peltier Technical Services
Tutorials and Custom Solutions
http://PeltierTech.com/
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Max McGovern wrote:

How do I create a trendline for the past 13 weeks on a 52 week chart?

Thanks,

Max McGovern