[R] Remove all box around a plot except bottom line (base graphics)

[Michael Dewey]

To make the plot clearer I have removed the axes but I wish to remove 
all the boz except the bottom horizontal line. Using the bty parameter 
does not seem to enable me to just leave the horizontal line at the 
foot. I can get the "l" version to remove everything except the left 
hand side and the base. I could also remove the entire box but that 
looks odd. Am I reading the documentation incorrectly or is it really 
impossible?

-- 
Michael