Hi, I'm trying to plot a series of lines without data point markers, except for the first data point in each line, which I want to also mark with an open circle. The following code accomplishes this for a single line: xyplot(yy ~ xx, panel=function(x, y){ panel.xyplot(x, y, col="black",type="l") panel.xyplot(x[1], y[1], col="black",type="p") }) However, I am trying to do this for every line for multiple lines in a panel, with several panels created by a 'by()' function (I do not want a grid of panels, I want separated panels). expt1 is a dataframe with seven factor columns, and x,y data coordinates in columns 8 and 9. I can appropriately create the grouped lines (but without the single data markers) with following code: by(expt1,expt1[,3:4],function(q) xyplot(q[,9] ~ q[,8], groups=q[,2], data=q, col="black", type="l")) But I am stuck putting the two goals together because I can't figure out how to get the group information to be properly passed to the custom panel function above. I suspect I may be going about this entirely the wrong way - I've been using R for all of 4 hours... Help appreciated. Russell -- Russell Wyeth Biology, St Francis Xavier University P.O. Box 5000 Antigonish, NS B2G 2W5 Canada Shipping: 1 West St. Antigonish, NS B2G 2W5 Canada http://people.stfx.ca/rwyeth/ Ph: 9028673886 Fx: 9028672389 Cell: 9023180250