I may give it a try. Do you plan on using the plugin only for functions? (only one possible y-value for one said x-value)

But first, here's what I think I would do, tell me if that's how you'd like it :

- In a Graph file, you'd need to have only, and in that order, the Axes, the Point Series and the Trend-line Function.

- You'd then select the Plugins menu and click on "Calculate Trendline Distance"

- This would create a new Point Series which would hold the distance between the original Point Series and the Trend-line Function, for each point in the original Point Series.

As a matter of sign convention, when the trendline is below the point, would you like the Distance to be a positive number?