vector that gives the minimum and maximum values for each level of a
shingle;
all minimum values are first, followed by all maximum values.
There are always an even number of
x values,
since they come in min/max pairs.
y
vertical coordinates of the rectangles.
...
graphical parameters can be passed to the line-drawing code.
By default, parameter
col is used from trellis parameter list
bar.fill.
SIDE EFFECTS:
panel.plot.shingle draws horizontal rectangles
to represent the range of
x values
in each level of a factor or shingle.