Phil -

You can have only one trendline per series, and the trendline analyzes
the entire series.  You have two options, and both require breaking up
the data somehow.

Option 1 is to chart Jan-June as one series, and July-Dec as another,
and run separate trendlines.

Option 2 is to keep the series intact, but in your worksheet perform
linear regression (look up LINEST, or SLOPE and INTERCEPT if it's a
simple first order line fit), then use the parameters to determine the
X,Y pairs at either end of the fitted line segments, and plot these as
separate series.

The first option os probably easier.

- Jon
-------
Jon Peltier, Microsoft Excel MVP
http://www.geocities.com/jonpeltier/Excel/index.html
_______