Hi,
If you want the chart to do this, then one way is to plot the "obvious
linear protion" as one series and the rest as another series and then select
the above portion and choose Chart, Add Trendline. You could also manually
calculate the trendline in the spreadsheet for the points that make up the
"obvious linear portion" using some combination of the LINEST, TREND, SLOPE
or INTERCEPT spreadsheet functions.
Then you would calculate and plot the trendline as a separate series. I
suspect you can find at least one example at Jon's website. Google Excel
Charts and look for Jon Peltier's sites.