Get margin line locations (mgp) in user coordinates

Question

I am trying to do some non-conventional plot labeling and would like a way to convert the line parameter in mtext and axis to user coo

Answer 1

The following should do the trick:

setup_plot()
abline(v=par('usr')[1] - (0:9) * 
         diff(grconvertX(0:1, 'inches', 'user')) * 
         par('cin')[2] * par('cex') * par('lheight'), 
       xpd=TRUE, lty=2)

par('cin')[2] * par('cex') * par('lheight') returns the current line height in inches, which we convert to user coordinates by multiplying by diff(grconvertX(0:1, 'inches', 'user')), the length of an inch in user coordinates (horizontally, in this case - if interested in the vertical height of a line in user coords we would use diff(grconvertY(0:1, 'inches', 'user'))).

This can be wrapped into a function for convenience as follows:

line2user <- function(line, side) {
  lh <- par('cin')[2] * par('cex') * par('lheight')
  x_off <- diff(grconvertX(0:1, 'inches', 'user'))
  y_off <- diff(grconvertY(0:1, 'inches', 'user'))
  switch(side,
         `1` = par('usr')[3] - line * y_off * lh,
         `2` = par('usr')[1] - line * x_off * lh,
         `3` = par('usr')[4] + line * y_off * lh,
         `4` = par('usr')[2] + line * x_off * lh,
         stop("side must be 1, 2, 3, or 4", call.=FALSE))
}

setup_plot()
abline(v=line2user(line=0:9, side=2), xpd=TRUE, lty=2)

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EDIT: An updated version of the function, which works with logged axes, is available here.