Plots multiple lines based on a vector x and a matrix y,
draws thin vertical lines connecting limits represented by columns of
y beyond the first. It is assumed that either (1) the second
and third columns of y represent lower and upper confidence
limits, or that (2) there is an even number of columns beyond the
first and these represent ascending quantiles that are symmetrically
arranged around 0.5.
a numeric matrix with number of rows equal to the number of
x elements
pos
when pos='left' the vertical lines are drawn, right
to left, to the left of the point (x, y[,1). Otherwise lines
are drawn left to right to the right of the point.
col
a color used to connect (x, y[,1]) pairs. The same
color but with transparency given by the alpha argument is
used to draw the vertical lines