Dear R-help subscribers, Many thanks for all the answers I received either by mail or through the list, and that were most helpful.
For the sake of the list records, I wanted to post the solution, I eventually could obtain, to the problem I have submitted. I am summarizing the question here. It was how to use the panel function "panel.curve" when the expression used in this function to to add a fitted line to the points in each panel depends not only on x, but also on the value of the conditioning variable for the considered panel. This solution is based on hints given to me by Deepayan Sarkar, it relies on the use of "packet.number" and "panel.number": xyplot(n ~ cg | di, data = myData, scale = list(y = "free", x = "free"), groups = bloc, as.table = T, xlab = "Cg", ylab = "N / ha", panel = function(x, y, subscripts, groups, packet.number = di, panel.number, ...) { panel.grid(h = -1, v = -1, col = "grey", lwd = 1, lty = 2) panel.curve(expr = f.fit(unique(rev(packet.number))[panel.number], a, b, a1, b1, x), n = 50, curve.type = "l", col = "lightblue", ...) panel.superpose(x, y, pch = c(1, 2), col = c("deeppink","blue"), panel.groups = "panel.xyplot", subscripts, groups) }, key = list( space = "top", transparent = TRUE, columns = 2, points = list( pch = c(1, 2), col = c("deeppink", "blue") ), text = list( c("bloc 3", "bloc 4"))), ) Some comments: - the arguments "packet.number = di, panel.number" must appear on the panel function prototype - a, b, a1, b1 are parameters set to a certain value before calling "xyplot" - "unique" is needed to reduce to 1 the multiple instances of the value of the conditioning variable di for a given panel (one for each point) - I was surprised by the need to use "rev" in order for the interval of the values to draw the curve f.fit to correspond to the right panel. Any further comment would be welcome Best regards Jean-Marc ---- Jean-Marc Ottorini LERFoB, UMR INRA-ENGREF 1092 email [EMAIL PROTECTED] INRA - Centre de Nancy voice +33-0383-394046 F54280 - Champenoux fax +33-0383-394034 France ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html