LTSpice Measurement on Plot
I don't think it's possible in LTspice itself, but you can check the LTspice user group at:  http://groups.yahoo.com/group/LTspice/ for a definitive answer. The way I do it is to put the cursor in the plot pane and click it, then click on Tools>Copy Bitmap to Clipboard, then launch Paint, paste the bitmap, and annotate away...