Excel ter graph question, please help?

As far as I'm aware, you can't get the Insert Trendline facility to use only part of the series plotted. So, your options are either to plot each part of the series as a separate series, which you can disguise by making the markers and line styles all the same - if your using connecting lines, you'll need to make the first point of series 2 the same as the last point of series 1 so you duplicate some data - or use the regression functions SLOPE(), INTERCEPT() and LINEST() on the spreadsheet instead of on the chart.