[R] using R interactively

[Terry J. Westley]

Thanks for your suggestions to my questions:
> 1) Is it possible to erase data points, legends, and/or text
>    from an R plot?

Several of you suggested to the method I finally chose: rewrite
the data point to be erased with the background color.

> 2) What's the best way to find the closest data point to
>    what locator() returns?

Thanks to Charles Berry for his suggestion on using the Euclidian
distance method.  I had to modify his idea to convert the position
of my data points to inches in order to account for non-square
plots and non-equal axes lengths.  Below is the code for a simple
test version of what I did.

--
Terry J. Westley, Principal Engineer
Veridian Engineering, Calspan Operations
P.O. Box 400, Buffalo, NY 14225
twestley at buffalo.veridian.com    http://www.veridian.com

#-----------------------------------------------------------------------
loc.prev <- list(x=NA, y=NA)

# Creates two vectors, x and y, of n runif
# data points and plots them -- this is just
# used for testing functions plotdist and
# closest.
#------------------------------------------
newplot <- function (n) {
  x <<- 1:n
  y <<- runif(n)
  plot(x, y, pch="o")
}

# Computes vector of distances between
# x0,y0 and x1,y1 data points.  Get usr
# and pin each time to account for user
# resizing the window.
#-----------------------------------------
plotdist <- function (x0, y0, x1, y1, usr=par("usr"), pin=par("pin")) {
  # xpi means X coordinates Per Inch
  xpi <- (usr[2] - usr[1]) / pin[1]
  ypi <- (usr[4] - usr[3]) / pin[2]
  (((x0-x1)/xpi)^2 + ((y0-y1)/ypi)^2)^0.5
}

# Determines which vector element in x,y is
# closest to lx, ly.
#------------------------------------------
closest <- function (x, y, lx, ly) {
  dist <- plotdist(x, y, lx, ly)
  dist.min <- min(dist)
  seq(along=x)[dist == dist.min]
}

# Let user click on a plot, then show
# details of the data point.
#------------------------------------
show <- function () {
  while (1) {

    # get location user clicked on and
    # determine index in x,y
    #---------------------------------
    l <- locator(1)
    index <- closest(x, y, l$x, l$y)

    # Erase marker on previous point,
    # redraw previous point and save
    # newly selected data point for
    # next erase.
    #-------------------------------
    if (!is.na(loc.prev$x)) {
      points(x=loc.prev$x, y=loc.prev$y,
        pch="X", col="white")
      points(x=loc.prev$x, y=loc.prev$y,
        pch="o", col="black")
    }
    loc.prev <- list(x=x[index], y=y[index])

    # Display a marker of which data point
    # the user selected and a legend with
    # details of the data.
    #-------------------------------------
    points(x=x[index], y=y[index], pch="X")
    legend (x=1, y=max(y),
      legend=paste("y[", index, "]=", y[index], sep=""))
  }
}

# demonstrate with a plot of 50 random numbers
#---------------------------------------------
newplot(50)
show()

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