Add a Straight Line to a HexPlot
This function adds one or more straight lines through the current
plot; it is the hexbin version of abline().
hexVP.abline(hvp, a = NULL, b = NULL, h = numeric(0), v = numeric(0),
             col = "black", lty = 1, lwd = 2, ...)| hvp | A hexViewport object that is currently on the active device | 
| a,b | the intercept and slope or if  | 
| h | the y-value for a horizontal line. | 
| v | the x-value for a vertical line. | 
| col, lty, lwd | line color, type and width. | 
| ... | further graphical parameters. | 
The first form specifies the line in intercept/slope form
(alternatively a can be specified on its own and is taken to
contain the slope and intercept in vector form).
The h= and v= forms draw horizontal and vertical
lines at the specified coordinates.
The coef form specifies the line by a vector containing the
slope and intercept.
lm is a regression object which contains reg$coef.  If it is
of length 1 then the value is taken to be the slope of a line
through the origin, otherwise, the first 2 values are taken to be
the intercept and slope.
Nicholas Lewin-Koh
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