Does anyone have an example of this?
I was able to do this yesterday by adding line series tags to my plotChart.
The Regression/Trend Line is calculated using the Least Squares Regression method. You can get all the information on what goes into the calculations for Least Squares Regression on wikipedia.
Then create an actionscript class (its not particularly complicated) that does the Least Squares Regression calculations on a series of x and y observations (that you used to create your plot chart).
Once you have calculated the Least Squares Regression data for your plot series, you add a line series to your plot chart. Each observation in the lineSeries data should use the x value (independant variable) from the plot data and the calculated y value from your Least Squares Regression class.