# function quartplot: A scatterplot for assessing the relationship between
# a continuous variable and a dichotomous variable.  Includes jitter for
# dichotomous variable

# OPTIONS:
# y = name of a vector containing 0s and 1s
# x = name of a vector containing a continous variable
# jitamount = amount of jitter above 0 and below 1 
# smoothspan = proportion of data used in the lowess smoother
# quartiles = 0 or 1: If 1, shows vertical lines at the 25, 50, and 75th
  
# NOTES:
# 1. Smoothed line uses real values, not jittered values.
# 2. Only y values of 0 and 1 are plotted.  Other values ignored.
# 3. Quartiles computed among observations where y= 0 or 1.

# CREATED by Ken Kleinman, 6/8/2009

quartplot <- function(x,y,jitamount,smoothspan,quantiles)
{
jittery <- jitter(y, amount=jitamount/2)
correction <- ifelse(y==0, jitamount/2, -jitamount/2)
jittery <- jittery + correction
plot(x,y,type="n")
points(x,jittery, pch=20, col="blue")
lines(lowess(x,y, f=smoothspan), col = "blue")
if (quantiles==1) {abline(v=quantile(x,c(.25,.5,.75)))}
}

# make some sample data
beta <- 0.5
xtest <- rnorm(1000,1,1)
linpred <- xtest * beta
prob <- exp(linpred)/(1 + exp(linpred))
runis <- runif(1000,0,1)
ytest <- ifelse(runis < prob,1,0)
quartplot(xtest,ytest,.05,.4,1)
quartplot(xtest,ytest,.05,.4,0)