# function to draw screeplots of cumulative
# eigenvalues in principal component analysis

screeplot<-function(mydata,cor=F,maxcomp=10) {
my.pc<-prcomp(mydata, scale=cor)
k<-min(dim(mydata),maxcomp)
x<-c(0:k)
y<-my.pc$sdev[1:k]*my.pc$sdev[1:k]
y<-c(0,y)
z<-100*cumsum(y)/sum(my.pc$sdev*my.pc$sdev)

plot(x,z,type="l",xlab="number of dimensions",
 cex.main=1.5, lwd=3, col="red",
 ylim=c(0,100),
 ylab="cumulative percentage of total variance",
  main="Scree plot of variances",
 xaxt="n", yaxt="n")
 
axis(1,at=x,lwd=2)
axis(2,at=c(0,20,40,60,80,100),lwd=2)
abline(a=100,b=0,lwd=2,lty="dashed",col="orange")
text(x,z,labels=x,cex=0.8,adj=c(1.2,-.1),col="blue")
}


# Examples of calls to it are
screeplot(mydata) # default uses covariance, maximum 10 components
screeplot(mydata,T) #  uses correlations, maximum 10 components
screeplot(mydata,maxcomp=7) # default use covariance, maximum 7 components
screeplot(mydata,T,maxcomp=8) # use correlations, maximum 8 components