Turning off the masking can have other undesirable effects. If you
still want to go that way, you might also need to set masksToBounds =
NO.
A better bet will be to change your plot range slightly to move the
limits of the plot inwards. You can limit the lengths of the axis and
grid lines to make it appear like your plot extends to the edge.
See this other thread for more suggestions:
http://groups.google.com/group/coreplot-discuss/browse_thread/thread/180ddb5624f2aa6c#
Eric