'' adds error bars to a bar plot. Typically, '' is
preceded by a call to 'barplot' that plots the bar graph and outputs the
x coordinates of the midpoints of the bars.  Schematically, the typical
use of '' looks like:
data = <vector of heights of successive bars>
x.midpoints <- barplot(data, ....)
error.bars(x.mid = x.midpoints, y.height = data, lower.bnd = ci.halfwidth, ...)
where 'data' is a vector or matrix of bar heights (means, medians, counts, etc.),
and 'ci.halfwidth' is a vector or matrix of the halfwidths (halfheights) of the
desired error bars.  Alternatively 'y.height' can be set to NA (default) if
the error bars are specified by vectors of y-coordinaes for 'lower.bnd'
and 'upper.bnd'.  The latter is useful when the coordinates for the
lower and upper ends of the error bars have already been computed, e.g.,
as the output of the 'res.params' function.
ARGUMENTS:
'x.mid' = x coordinate midpoints of the bars. 'barplots' outputs these
midpoints.
'y.height' = vector or matrix of y coordinates (heights) for the bars.  Typically,
'y.height' is identical to the data vector or matrix that was previously inputted
to 'barplot'.
'lower.bnd' is either the half the width of the error bar, represented as a
positive number, or the coordinates of the lower bound (in this case,
'y.height' must be NA.
'upper.bnd' is by default the same as 'lower.bnd', but it can be set to other
values for asymmetic error bars.  If 'y.height' is NA, 'upper.bnd' to be set
to the y-coordinates of the upper bound.
'end.width' is the width of the end line at the bottom and top
of the error bar.  The default is NA in which case 'end.width' is
set to 0.4% of the horizontal plotting area.
'lwd.f' specifies the line width of the error bars.
'error.bars' operates by passing the parameters to a call to 'arrows' which
draws the error bars.  Other parameters can be passed to 'arrows' such as
color ('col').

'euro' = amount of Euros to be converted to US dollars.
'dol' = amount of US dollars to be converted to Euros.
'e.to.d' = number of dollars that equal 1 Euro
'd.to.e' = number of Euros that equal 1 dollar