How do I compute or plot trend lines?

How do I compute or plot trend lines?

First, You need a column on your spreadsheet to hold average values. Averages can be made by using one of these formulas:
=SUM(<cell range>/<number of cells>)
=AVERAGE(<cell range>)

Now, on to graphing:

You can do trendlines in Calc, but you can't show the equation of the line. To do that you need to do a linear array function.

Trendline: You have to have an xy chart to make a trendline (change the chart type). When you have a chart, select the data points, and then right click and select 'Object properties'. In the statistics tab, select the regression picture (none, linear, etc.) that you want.

OpenOffice.org Version 1, Calc only really does linear.