[R] Scaling in plot function

[Marc Schwartz]

On Sun, 2006-03-12 at 00:09 +0100, Prasanna BALAPRAKASH wrote: 
> Dear R helpers,
> 
> I have a vector of 500 numbers and I need to plot them in such a way  
> that the first 250 values should occupy 80% of the plot and the  
> remaining ones should take 20%. More precisely, x axis ranges form  
> 1:500 and the idea is to give the snapshot of the first 250 values. I  
> tried  "axis()" and log="x", but I am not getting the required output.
> 
> 
> Thanks
> Prasanna


There may be an easier way to do this, but here are two approaches that
come to mind.  The first does this in a single plot region, the second
uses layout() to allocate space within the plot region 80/20 to two
separate plots.


1. First approach using a single plot region

# Create an initial set of 250 x vals
# from 0 to 0.8
x250.8 <- seq(0, 0.8, length = 250)

# Create a second set of 250 from 0.8 to 1.0
x250.2 <- seq(0.8, 1, length = 250)

# Concatenate the two vectors to form
# the 500 x axis vals
x.axis <- c(x250.8, x250.2)


# Now do the plot, using the 500 vals
# for the x axis values
# Draw 1st 250 with red
# Draw second 250 with green
plot(x.axis, rnorm(500), pch = 20, 
     col = c(rep("red", 250), rep("green", 250)),
     xaxt= "n")


# Now set up x axis labels from 0 to 250
# by 50 and then at 500
axis(1, at = c(seq(0, 0.8, length = 6), 1), 
     labels = c(seq(0, 250, 50), 500))


See ?seq and ?plot.default for more information.



2. Second approach using layout()

# Set a matrix of 2 cols
# Allocate 80% to 1 and 20% to 2
layout(matrix(1:2, ncol = 2), c(4, 1))

# Set margins to adjust distance
# between plots
par(mar = c(5, 2, 3, 1))

# Do plot of 1st 250 in red
# set ylim to match for both plots
# Set xlab so no output
plot(rnorm(250), ylim = c(-4, 4),
     pch = 20, col = "red", xlab = "")

# Set margins to adjust distance
# between plots
par(mar = c(5, 1, 3, 2))

# Do 2nd 250 in green
# Set xaxt so x axis is not drawn
# Set xlab so no output
plot(rnorm(250), ylim = c(-4, 4), yaxt = "n",
     pch = 20, xaxt = "n", col = "green", xlab = "")

# Now do x axis
axis(1, at = c(0, 250), labels = c(250, 500))

# Now add y axis on right plot
axis(4)

# Set outer margins for common labels
par(oma = c(3, 0, 3, 0))

# Do title
mtext(side = 3, "Example of using layout()", outer = TRUE,
      line = 1, cex = 1.5)

# Do common x axis label
mtext(side = 1, "Index", outer = TRUE,
      line = 1, cex = 1)


See ?layout, ?mtext and ?par for more information.


I think that in general, I would recommend the second approach, since it
emphasizes the separation between the two sets of 250. However, it may
be dependent upon your actual data.

HTH,

Marc Schwartz