On Wed, 23 Mar 2011, Walt wrote: Formulas entered via the GUI graph editor are fed directly to gnuplot, which doesn't know about "l_Price". However, (a) You can add the y-mean by looking up that variable's summary statistics in gretl and using the graph editor to add a line at that value. For example, if the y-mean is 3.65, just enter a "formula" of "3.65". (b) Adding the x-mean (a vertical line) is more awkward and you'll probably have to edit the gnuplot commands. (Save the graph "as an icon" then right-click the icon.) One way is to append a plot "line" using a single data-point, with x-value equal to the x-mean and y-value equal to the natural height of the graph. Here's an example, based on a scatter plot of price against sqft from gretl's data4-1: Before addition of vertical line: plot \ '-' using 1:($2) notitle w points, \ 52.35090729 + 0.1387503195*x title "Y = 52.4 + 0.139X" w lines, \ 317.49 title "mean(price)" w lines lt 3 1065 199.9 1254 228 [...] e After addition: plot \ '-' using 1:($2) notitle w points, \ 52.35090729 + 0.1387503195*x title "Y = 52.4 + 0.139X" w lines, \ 317.49 title "mean(price)" w lines lt 3, \ '-' using 1:2 title "mean(sqft)" w impulses lt 3 1065 199.9 1254 228 [...] e 1910.9 550 e In gnuplot "e" on a line by itself signals the end of an inline data block. Allin Cottrell