This function plots the output from function bf.c.o.
draw.bf(obj,line.lwd=2, ...)is a list with two elements, the vectors x and y to be
plotted in a scatterplot, which are produced by function bf.c.o.
The first element is the values of \(M\) to go on the x-axis,
and the second element is the Bayes factors calculated by
bf.c.o.
graphical parameter; the relative thickness of the plotted line
additional graphical parameters for the overall plot