R/powerplot.R

In DAAG: Data Analysis and Graphics Data and Functions

powerplot <-
function (expr = "x^2", xlab = "x", ylab = "y", ...) 
{
    invtxt <- switch(expr, `x^2` = "sqrt(y)", `x^4` = "y^0.25", 
        `exp(x)` = "log(x)", `sqrt(x)` = "y^2", `x^0.25` = "y^4", 
        `log(x)` = "exp(y)")
    x <- (1:60)/6
    y <- eval(parse(text = expr))
    form <- formula(paste("~", expr))
    dy <- deriv(form, "x")
    x0 <- min(x) + diff(range(x)) * 0.4
    y0 <- eval(parse(text = expr), list(x = x0))
    b <- eval(dy, list(x = x0))
    b <- attr(b, "gradient")
    plot(x, y, type = "n", xlab = "", ylab = "", ...)
    lines(x, y, type = "l", lwd = 2, col = 2)
    chh <- par()$cxy[2]
    theta <- atan(b * diff(par()$usr[1:2])/diff(par()$usr[3:4])) * 
        180/pi
    mtext(side = 1, line = 2.5, xlab, cex = 1)
    mtext(side = 2, line = 2.5, ylab, cex = 1)
    funexpr <- parse(text = paste("y ==", expr))
    text(x0, y0 + chh/2, funexpr, srt = theta, cex = 1.5)
    invexpr <- parse(text = invtxt)[[1]]
    titletxt <- substitute(paste(tx, tilde(y) == invexpr), list(tx = "Replace y by ", 
        invexpr = invexpr))
    mtext(side = 3, line = 0.5, titletxt)
}