Draw a rectangle surrounding text in a plot

textbox.R

# All arguments as in `graphics::text`, except pad which specifies padding (optionally 
# for vertical and horizontal) around the text as a fraction of a character width.
# `...` is passed to `rect`
textbox <-
function(x, y, labels, adj = NULL,
         pos = NULL,
         offset = 0.5,
         pad = c(0, 0),
         vfont = NULL,
         cex = par("cex"),
         font = NULL, ...) {
    xy <- xy.coords(x, y, recycle = TRUE, setLab = FALSE)
    n <- length(xy$x)
    if(is.call(labels)) labels <- as.expression(labels)
    pad <- as.numeric(rep(pad, length.out = 2L))
    if(is.null(pos))
        pos <- NA
    
    if(is.null(font))
         font <- par("font")
    
    u2ix <- function(x) grconvertX(x, from = "user", to = "inch")
    u2iy <- function(x) grconvertY(x, from = "user", to = "inch")
    i2ux <- function(x) grconvertX(x, from = "in", to = "user")
    i2uy <- function(x) grconvertY(x, from = "in", to = "user")
    
    # from now on all is in inches
    x <- u2ix(xy$x)
    y <- u2iy(xy$y)
    offset <- grconvertX(offset, from = "chars", to = "in")
    pad <- grconvertX(pad, from = "chars", to = "in")

    adj <- if (is.null(adj)) {
        c(.5, .5)
    } else if (is.numeric(adj)) {
        if (length(adj) == 0) {
            c(.5, .5)
        } else if (length(adj) == 1L) {
            c(adj, .5)
        } else {
            adj[1L:2L]
        }
    } else stop("invalid 'adj' value")
    
    # values taken from R source src/library/grDevices/src/dev*.c
    yCharOffset <- switch(names(dev.cur()),
        windows = 0.40,
        "xfig" = , "pdf" = , "postscript" = 0.3333,
         pictex =  0,
         #CAIRO
         "png" =, "jpeg"=, "svg"=, "png"=,
         "cairo_pdf"=, "cairo_ps"=, "tiff"=, "bmp"=  0.3333,
         "null device" = stop("no device is open"), 
            stop("unknown device")
         )
    

    z <- .mapply(function(x, y, s, pos, cex, font, pad, ...) {
        switch(pos, { # 1 bottom
            y <- y - offset
            adj <- c(0.5, 1 - (0.5 - yCharOffset))
        }, { # 2 left
            x <- x - offset
            adj <- c(1, yCharOffset)
        }, {  # 3 top
            y <- y + offset
            adjx <- 0.5
            adjy <- 0
            adj <- c(.5, 0)
        }, { # 4 right
            x <- x + offset
            adj <- c(0, yCharOffset)
        }, { })
        w <- strwidth(s, cex = cex, units = "in", font = font, vfont = vfont)
        h <- strheight(s, cex = cex, units = "in", font = font, vfont = vfont)
        x <- x - (adj[1L] * w)
        y <- y - (adj[2L] * h)
        c(x - pad[1L], y - pad[2L], x + w + pad[1L], y + h + pad[2L])
    },
        list(x = x, y = y, s = labels, pos = pos, cex = cex,
             font = font, ...),
        MoreArgs = list(pad = pad))
    z <- do.call("rbind", z)
    
    
    rect(i2ux(z[, 1L]), i2uy(z[, 2L]),
         i2ux(z[, 3L]), i2uy(z[, 4L]),
         ...) 
    
}

lab <- expression(
 {f * minute}(x),
 hat(beta) == (X^t * X)^{-1} * X^t * y,
 bar(x) == sum(frac(x[i], n), i==1, n),
 paste(frac(1, sigma*sqrt(2*pi)), " ",
         plain(e)^{frac(-(x-mu)^2, 2*sigma^2)}))

offset <- 3
x <- 0
y <- 0
plot(0, 0, xlim = c(-8, 8), ylim = c(-2, 2))
textbox(x, y, lab, cex =  1:4, pos = 1:4, offset = offset,
            pad = .4,
            col = "#0080ff99", border = NA,
            xpd = NA)
# recycling behaviour is different between text and textbox
text(x, y, rep(lab, 4), cex = 1:4, pos = 1:4, offset = offset,
     xpd = NA)