Draw a polygon on an existing xts plot by specifying a time series of y coordinates. The xts index is used for the x coordinates and the first two columns are the upper and lower y coordinates, respectively.
addPolygon(x, y = NULL, main = "", on = NA, col = NULL, ...)
an xts object to plot. Must contain 2 columns for the upper and lower y coordinates for the polygon. The first column is interpreted as the upper y coordinates and the second column as the lower y coordinates.
NULL, not used
main title for a new panel if drawn.
panel number to draw on. A new panel will be drawn if
color palette to use, set by default to rational choices.
passthru parameters to
Based on code by Dirk Eddelbuettel from http://dirk.eddelbuettel.com/blog/2011/01/16/