R change title size in legend

Question

I use the R base command "legend" to add a legend to a plot and I want the title of this legend to be bigger. In the help it says:

cex: character expansion factor relative to current ‘par("cex")’. Used for text, and provides the default for ‘pt.cex’ and ‘title.cex’.

But trying to change title.cex doesn't work and throw a unused argument error.

Here an example:

plot(1:10,10:1)
legend("topright",title="I need that bigger",legend="points",pch=21)

Answer 1

Since R 4.2.0, you can use the argument title.cex:

plot(1:10,10:1)
legend("topright",title="I need that bigger",legend="points", pch=21, title.cex = 2)

Answer 2

I found an answer here saying that:

Looking at the code for legend, it looks like the same 'cex' value is used for the text in the legend as the title.

And they say to change this:

text2(left + w/2, top - ymax, labels = title, adj = c(0.5,0), cex = cex, col = title.col)

With this:

text2(left + w/2, top - ymax, labels = title, adj = c(0.5,0), cex = title.cex, col = title.col)

I may end up doing this but if it's the only solution it looks to me like a bug, no?

Edit: This answer works indeed, thought the code of legend slightly changed in my version of R I changed this:

text2(left + w * title.adj, top - ymax, labels = title, adj = c(title.adj, 0), cex = cex, col = title.col)

by:

text2(left + w * title.adj, top - ymax, labels = title, adj = c(title.adj, 0), cex = title.cex, col = title.col)

and added: title.cex = cex in the arguments of legend. It's working, though the size of the box surrounding the legend should be recalulate as if the title is too big it goes outside the box.

For those that just want to copy paste a version of legend that accept title.cex here it is:

legend  <- function (x, y = NULL, legend, fill = NULL, col = par("col"), 
    border = "black", lty, lwd, pch, angle = 45, density = NULL, 
    bty = "o", bg = par("bg"), box.lwd = par("lwd"), box.lty = par("lty"), 
    box.col = par("fg"), pt.bg = NA, cex = 1, pt.cex = cex, pt.lwd = lwd, 
    xjust = 0, yjust = 1, x.intersp = 1, y.intersp = 1, adj = c(0, 
        0.5), text.width = NULL, text.col = par("col"), text.font = NULL, 
    merge = do.lines && has.pch, trace = FALSE, plot = TRUE, 
    ncol = 1, horiz = FALSE, title = NULL, inset = 0, xpd, title.cex = cex,title.col = text.col, 
    title.adj = 0.5, seg.len = 2) 
{
    if (missing(legend) && !missing(y) && (is.character(y) || 
        is.expression(y))) {
        legend <- y
        y <- NULL
    }
    mfill <- !missing(fill) || !missing(density)
    if (!missing(xpd)) {
        op <- par("xpd")
        on.exit(par(xpd = op))
        par(xpd = xpd)
    }
    title <- as.graphicsAnnot(title)
    if (length(title) > 1)
        stop("invalid 'title'")
    legend <- as.graphicsAnnot(legend)
    n.leg <- if (is.call(legend))
        1
    else length(legend)
    if (n.leg == 0)
        stop("'legend' is of length 0")
    auto <- if (is.character(x))
        match.arg(x, c("bottomright", "bottom", "bottomleft",
            "left", "topleft", "top", "topright", "right", "center"))
    else NA
    if (is.na(auto)) {
        xy <- xy.coords(x, y)
        x <- xy$x
        y <- xy$y
        nx <- length(x)
        if (nx < 1 || nx > 2) 
            stop("invalid coordinate lengths")
    }
    else nx <- 0
    xlog <- par("xlog")
    ylog <- par("ylog")
    rect2 <- function(left, top, dx, dy, density = NULL, angle, 
        ...) {
        r <- left + dx
        if (xlog) {
            left <- 10^left
            r <- 10^r
        }
        b <- top - dy
        if (ylog) {
            top <- 10^top
            b <- 10^b
        }
        rect(left, top, r, b, angle = angle, density = density, 
            ...)
    }
    segments2 <- function(x1, y1, dx, dy, ...) {
        x2 <- x1 + dx
        if (xlog) {
            x1 <- 10^x1
            x2 <- 10^x2
        }
        y2 <- y1 + dy
        if (ylog) {
            y1 <- 10^y1
            y2 <- 10^y2
        }
        segments(x1, y1, x2, y2, ...)
    }
    points2 <- function(x, y, ...) {
        if (xlog) 
            x <- 10^x
        if (ylog) 
            y <- 10^y
        points(x, y, ...)
    }
    text2 <- function(x, y, ...) {
        if (xlog) 
            x <- 10^x
        if (ylog) 
            y <- 10^y
        text(x, y, ...)
    }
    if (trace) 
        catn <- function(...) do.call("cat", c(lapply(list(...), 
            formatC), list("\n")))
    cin <- par("cin")
    Cex <- cex * par("cex")
    if (is.null(text.width)) 
        text.width <- max(abs(strwidth(legend, units = "user", 
            cex = cex, font = text.font)))
    else if (!is.numeric(text.width) || text.width < 0) 
        stop("'text.width' must be numeric, >= 0")
    xc <- Cex * xinch(cin[1L], warn.log = FALSE)
    yc <- Cex * yinch(cin[2L], warn.log = FALSE)
    if (xc < 0) 
        text.width <- -text.width
    xchar <- xc
    xextra <- 0
    yextra <- yc * (y.intersp - 1)
    ymax <- yc * max(1, strheight(legend, units = "user", cex = cex)/yc)
    ychar <- yextra + ymax
    if (trace) 
        catn("  xchar=", xchar, "; (yextra,ychar)=", c(yextra, 
            ychar))
    if (mfill) {
        xbox <- xc * 0.8
        ybox <- yc * 0.5
        dx.fill <- xbox
    }
    do.lines <- (!missing(lty) && (is.character(lty) || any(lty > 
        0))) || !missing(lwd)
    n.legpercol <- if (horiz) {
        if (ncol != 1) 
            warning(gettextf("horizontal specification overrides: Number of columns := %d", 
                n.leg), domain = NA)
        ncol <- n.leg
        1
    }
    else ceiling(n.leg/ncol)
    has.pch <- !missing(pch) && length(pch) > 0
    if (do.lines) {
        x.off <- if (merge) 
            -0.7
        else 0
    }
    else if (merge) 
        warning("'merge = TRUE' has no effect when no line segments are drawn")
    if (has.pch) {
        if (is.character(pch) && !is.na(pch[1L]) && nchar(pch[1L], 
            type = "c") > 1) {
            if (length(pch) > 1) 
                warning("not using pch[2..] since pch[1L] has multiple chars")
            np <- nchar(pch[1L], type = "c")
            pch <- substr(rep.int(pch[1L], np), 1L:np, 1L:np)
        }
        if (!is.character(pch)) 
            pch <- as.integer(pch)
    }
    if (is.na(auto)) {
        if (xlog) 
            x <- log10(x)
        if (ylog) 
            y <- log10(y)
    }
    if (nx == 2) {
        x <- sort(x)
        y <- sort(y)
        left <- x[1L]
        top <- y[2L]
        w <- diff(x)
        h <- diff(y)
        w0 <- w/ncol
        x <- mean(x)
        y <- mean(y)
        if (missing(xjust)) 
            xjust <- 0.5
        if (missing(yjust)) 
            yjust <- 0.5
    }
    else {
        h <- (n.legpercol + (!is.null(title))) * ychar + yc
        w0 <- text.width + (x.intersp + 1) * xchar
        if (mfill) 
            w0 <- w0 + dx.fill
        if (do.lines) 
            w0 <- w0 + (seg.len + x.off) * xchar
        w <- ncol * w0 + 0.5 * xchar
        if (!is.null(title) && (abs(tw <- strwidth(title, units = "user", 
            cex = cex) + 0.5 * xchar)) > abs(w)) {
            xextra <- (tw - w)/2
            w <- tw
        }
        if (is.na(auto)) {
            left <- x - xjust * w
            top <- y + (1 - yjust) * h
        }
        else {
            usr <- par("usr")
            inset <- rep_len(inset, 2)
            insetx <- inset[1L] * (usr[2L] - usr[1L])
            left <- switch(auto, bottomright = , topright = , 
                right = usr[2L] - w - insetx, bottomleft = , 
                left = , topleft = usr[1L] + insetx, bottom = , 
                top = , center = (usr[1L] + usr[2L] - w)/2)
            insety <- inset[2L] * (usr[4L] - usr[3L])
            top <- switch(auto, bottomright = , bottom = , bottomleft = usr[3L] + 
                h + insety, topleft = , top = , topright = usr[4L] - 
                insety, left = , right = , center = (usr[3L] + 
                usr[4L] + h)/2)
        }
    }
    if (plot && bty != "n") {
        if (trace) 
            catn("  rect2(", left, ",", top, ", w=", w, ", h=", 
                h, ", ...)", sep = "")
        rect2(left, top, dx = w, dy = h, col = bg, density = NULL, 
            lwd = box.lwd, lty = box.lty, border = box.col)
    }
    xt <- left + xchar + xextra + (w0 * rep.int(0:(ncol - 1), 
        rep.int(n.legpercol, ncol)))[1L:n.leg]
    yt <- top - 0.5 * yextra - ymax - (rep.int(1L:n.legpercol, 
        ncol)[1L:n.leg] - 1 + (!is.null(title))) * ychar
    if (mfill) {
        if (plot) {
            if (!is.null(fill)) 
                fill <- rep_len(fill, n.leg)
            rect2(left = xt, top = yt + ybox/2, dx = xbox, dy = ybox, 
                col = fill, density = density, angle = angle, 
                border = border)
        }
        xt <- xt + dx.fill
    }
    if (plot && (has.pch || do.lines)) 
        col <- rep_len(col, n.leg)
    if (missing(lwd) || is.null(lwd)) 
        lwd <- par("lwd")
    if (do.lines) {
        if (missing(lty) || is.null(lty)) 
            lty <- 1
        lty <- rep_len(lty, n.leg)
        lwd <- rep_len(lwd, n.leg)
        ok.l <- !is.na(lty) & (is.character(lty) | lty > 0) & 
            !is.na(lwd)
        if (trace) 
            catn("  segments2(", xt[ok.l] + x.off * xchar, ",", 
                yt[ok.l], ", dx=", seg.len * xchar, ", dy=0, ...)")
        if (plot) 
            segments2(xt[ok.l] + x.off * xchar, yt[ok.l], dx = seg.len * 
                xchar, dy = 0, lty = lty[ok.l], lwd = lwd[ok.l], 
                col = col[ok.l])
        xt <- xt + (seg.len + x.off) * xchar
    }
    if (has.pch) {
        pch <- rep_len(pch, n.leg)
        pt.bg <- rep_len(pt.bg, n.leg)
        pt.cex <- rep_len(pt.cex, n.leg)
        pt.lwd <- rep_len(pt.lwd, n.leg)
        ok <- !is.na(pch)
        if (!is.character(pch)) {
            ok <- ok & (pch >= 0 | pch <= -32)
        }
        else {
            ok <- ok & nzchar(pch)
        }
        x1 <- (if (merge && do.lines) 
            xt - (seg.len/2) * xchar
        else xt)[ok]
        y1 <- yt[ok]
        if (trace) 
            catn("  points2(", x1, ",", y1, ", pch=", pch[ok], 
                ", ...)")
        if (plot) 
            points2(x1, y1, pch = pch[ok], col = col[ok], cex = pt.cex[ok], 
                bg = pt.bg[ok], lwd = pt.lwd[ok])
    }
    xt <- xt + x.intersp * xchar
    if (plot) {
        if (!is.null(title)) 
            text2(left + w * title.adj, top - ymax, labels = title, 
                adj = c(title.adj, 0), cex = title.cex, col = title.col)
        text2(xt, yt, labels = legend, adj = adj, cex = cex, 
            col = text.col, font = text.font)
    }
    invisible(list(rect = list(w = w, h = h, left = left, top = top), 
        text = list(x = xt, y = yt)))
}