# fields, Tools for spatial data
# Copyright 2004-2011, Institute for Mathematics Applied Geosciences
# University Corporation for Atmospheric Research
# Licensed under the GPL -- www.gpl.org/licenses/gpl.html
"arrow.plot" <- function(a1, a2, u = NA, v = NA, arrow.ex = 0.05, 
    xpd = TRUE, true.angle = FALSE, arrowfun = arrows, ...) {
    if (is.matrix(a1)) {
        x <- a1[, 1]
        y <- a1[, 2]
    }
    else {
        x <- a1
        y <- a2
    }
    if (is.matrix(a2)) {
        u <- a2[, 1]
        v <- a2[, 2]
    }
    ucord <- par()$usr
    arrow.ex <- arrow.ex * min(ucord[2] - ucord[1], ucord[4] - 
        ucord[3])
    if (true.angle) {
        pin <- par()$pin
        r1 <- (ucord[2] - ucord[1])/(pin[1])
        r2 <- (ucord[4] - ucord[3])/(pin[2])
    }
    else {
        r1 <- r2 <- 1
    }
    u <- u * r1
    v <- v * r2
    maxr <- max(sqrt(u^2 + v^2))
    u <- (arrow.ex * u)/maxr
    v <- (arrow.ex * v)/maxr
    invisible()
    old.xpd <- par()$xpd
    par(xpd = xpd)
    arrowfun(x, y, x + u, y + v, ...)
    par(xpd = old.xpd)
}