The position of the legend, either at the "top" or the "right" unlesss positioned inside the plot. If you want the legend to be positioned inside the plot then you have to provide a list with the same x & y qualities as legend. For instance if you want the legend to be positioned at the top right corner then use pos = list("topright") - this is equivalent to pos = list(x=1, y=1). If you want to have a distance from the edge of the graph then add a inset to the list, e.g. pos = list("topright", "inset"=.1) - the inset should be either a unit element or a value between 0 and 1. The default is to have the boxes aligned vertical, if you want them to be in a line then you can specify the "align" option, e.g. pos = list("topright", "inset"=.1, "align"="horizontal")