A common problem with scatterplots is the overlapping of dots when a variable is quantitative and not qualitative. (see graph on the left). One of the solution consists to use jitter (see graph on the right). There is the code to realize it:
# Create data
a=rnorm(200 , mean=10 , sd=9)
Y=c( a-3 , a , a+5, a+10 , a+20 )
# plot without jitter
# plot with jitter
plot(jitter(X), Y , pch=20, col=rgb(0.1, 0.2, 0.8, 0.3) , xlab="X", ylab="Y", bty="n" )
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