Histogram and density plot

Problem

You want to make a histogram or density plot.

Solution

Some sample data: these two vectors contain 200 data points each:

set.seed(1234)
rating  <- rnorm(200)
# -1.207065749  0.277429242  ...  0.162654708 -2.078237542

rating2 <- rnorm(200, mean=.8)
# 1.28522682  1.49676878  ...  0.81464312 -0.11648914

When plotting multiple groups of data, some graphing routines require a data frame with one column for the grouping variable and one for the measure variable.

# Make a column to indicate which group each value is in
cond <- factor( rep(c("A","B"), each=200) )

data <- data.frame(cond, rating = c(rating,rating2))
# cond     rating
#    A -1.2070657
#    A  0.2774292
#    A  1.0844412
#   ...
#    B  1.3388331
#    B  0.8146431
#    B -0.1164891

# Histogram
hist(rating)

# Kernel density plot
plot(density(rating))

# Use 8 bins (this is only approximate - it places boundaries on nice round numbers)
# Make it light blue #CCCCFF
# Instead of showing count, make area sum to 1, (freq=FALSE)
hist(rating, breaks=8, col="#CCCCFF", freq=FALSE)

# Put breaks at every 0.6
boundaries <- seq(-3, 3.6, by=.6);
# -3.0 -2.4 -1.8 -1.2 -0.6  0.0  0.6  1.2  1.8  2.4  3.0  3.6
hist(rating, breaks=boundaries)

Multiple groups with kernel density plots.

This code is from: http://onertipaday.blogspot.com/2007/09/plotting-two-or-more-overlapping.html

plot.multi.dens <- function(s)
{
    junk.x = NULL
    junk.y = NULL
    for(i in 1:length(s)) {
        junk.x = c(junk.x, density(s[[i]])$x)
        junk.y = c(junk.y, density(s[[i]])$y)
    }
    xr <- range(junk.x)
    yr <- range(junk.y)
    plot(density(s[[1]]), xlim = xr, ylim = yr, main = "")
    for(i in 1:length(s)) {
        lines(density(s[[i]]), xlim = xr, ylim = yr, col = i)
    }
}

# the input of the following function MUST be a numeric list
plot.multi.dens( list(rating, rating2))

The sm package also includes a way of doing multiple density plots. The data must be in a data frame.

library(sm)
sm.density.compare(data$rating, data$cond)
# Add a legend (the color numbers start from 2 and go up)
legend("topright", levels(data$cond), fill=2+(0:nlevels(data$cond)))