add_mtext: new 'adj' argument to align text along the margin

R/plot_add.R

@@ -194,31 +194,36 @@ add_abline <- function(h=NULL, v=NULL, ...) {
 }
 
 
-add_mtext <- function(text, side=3, line=0, ...) {
+add_mtext <- function(text, side=3, line=0, adj=0.5, ...) {
 
 	stopifnot(side %in% 1:4)
 
 	p <- unlist(get.clip())
 	h <- graphics::strheight(text, units = "user", ...)	
+	adj <- c(adj[1], 0.5)
 
 	srt <- 0
-	if (side==1) {
-		x <- mean(p[1:2])
+	if (side == 1) {
+		x <- p[1] + adj[1] * (p[2] - p[1])
 		y <- p[3] - h - line * h
-	} else 	if (side==2) {
-		x <- p[1] -1.25 * h - line * h
-		y <- mean(p[3:4])
+		a <- adj
+	} else 	if (side == 2) {
+		x <- p[1] - 1.25 * h - line * h
+		y <- p[3] + adj[1] * (p[4] - p[3])
 		srt <- 90
-	} else 	if (side==3) {
-		x <- mean(p[1:2])
+		a <- adj
+	} else 	if (side == 3) {
+		x <- p[1] + adj[1] * (p[2] - p[1])
 		y <- p[4] + h + line * h
+		a <- adj
 	} else {
 		x <- p[2] + 1.25 * h + line * h
-		y <- mean(p[3:4])
+		y <- p[3] + adj[1] * (p[4] - p[3])
 		srt <- 270
+		a <- c(1 - adj[1], adj[2])
 	} 
 
-	text(x=x, y=y, labels=text, xpd=TRUE, srt=srt, ...)
+	text(x=x, y=y, labels=text, xpd=TRUE, srt=srt, adj=a, ...)
 }
 
 

man/add_mtext.Rd

@@ -9,13 +9,14 @@ Similar to \code{\link[graphics]{mtext}} allowing adding a text to the margins o
 }
 
 \usage{
-add_mtext(text, side=3, line=0, ...)
+add_mtext(text, side=3, line=0, adj=0.5, ...)
 }
 
 \arguments{
   \item{text}{character or expression vector specifying the text to be written}
   \item{side}{ integer indicating the margin to use (1=bottom, 2=left, 3=top, 4=right)}
-  \item{line}{ numeric to move the text in or outwards.}
+  \item{line}{ numeric to move the text in or outwards}
+  \item{adj}{numeric between 0 and 1 to align the text along the margin. Use 0 for left (sides 1 and 3) or bottom (sides 2 and 4), 1 for right or top, and 0.5, the default, to center it}
   \item{...}{arguments passed to \code{\link{text}}}
 }
 
@@ -29,6 +30,14 @@ r <- rast(f)
 plot(r, axes=FALSE, legend=FALSE)
 add_box()
 for (i in 1:4) add_mtext("margin text", i, cex=i, col=i, line=2-i)
+
+# align the text along the margin
+plot(r, axes=FALSE, legend=FALSE)
+add_box()
+for (i in 1:4) {
+	add_mtext("start", i, adj=0, col="blue")
+	add_mtext("end", i, adj=1, col="red")
+}
 }
 
 \keyword{methods}