#======================================================================
# Figure 12.4.R.
# R script to draw scatterplot of Old Faithful data with three
# candidate prediction lines overlayed.
#
# Data file should have two columns:  response variable to be
# predicted labeled "y" and predictor variable labeled "x".
# Change the R working directory to the location of the data file.
#====================================================================== 

Geyser=read.table("old_faithful.txt",header=TRUE)  # Change file name if
                                                   #  necessary.
attach(Geyser)

plot(x,y,type="p",xlab="duration of eruption (minutes)",
  ylab="time until next eruption (minutes)")  #Scatterplot of data.

b=numeric(2)  # Set up vector to hold intercept and slope.

# Calculate and plot first line.
b[1]=25   # Intercept.
b[2]=18   # Slope.
yhat1=b[1]+b[2]*min(x)
yhat2=b[1]+b[2]*max(x)
yhat=rbind(yhat1,yhat2)
xvals=rbind(min(x),max(x))
points(xvals,yhat,type="l",lty=2)

# Calculate and plot second line.
b[1]=50
b[2]=8
yhat1=b[1]+b[2]*min(x)
yhat2=b[1]+b[2]*max(x)
yhat=rbind(yhat1,yhat2)
xvals=rbind(min(x),max(x))
points(xvals,yhat,type="l",lty=2)

# Calculate and plot third line.
b[1]=33
b[2]=10
yhat1=b[1]+b[2]*min(x)
yhat2=b[1]+b[2]*max(x)
yhat=rbind(yhat1,yhat2)
xvals=rbind(min(x),max(x))
points(xvals,yhat,type="l",lty=2)

detach(Geyser)