#First, enter values for matrix
veg<-matrix(c(60,30,5,5,0,50,25,20,3,2,30,10,20,20,20,10,20,30,30,10),nrow=5)
#You could have also read in as read.table and used as.matrix function

#Check to make sure your matrix is correct
veg
#Add column and rownames
colnames(veg)<-c("1-2","3-4","5-6","7-8")
rownames(veg)<-c("OBL","FACW","FAC","FACU","UPL")
#Check to make sure your matrix is correct
veg

#Use xlmim function to lengthen x-axis and place legend
barplot(veg, legend.text=rownames(veg),xlim=c(0,6.5),xlab="Position on Transect (m)",ylab="Cover (%)")
#Add a line where 50% is; use abline function
abline(h=50, lty=3)
#Add a pont marker where split occurs
points(2.5,0,pch=17, cex=2)