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R: Graphing with matplot()


matplot() is a R function that you can use to make easy graphs in R.

Let’s start by creating a data set.

We are going to create a 5 x 10 matrix representing 10 quiz and test grades for 5 students

x <- sample(50:100,50, T)
# convert to a 5x10 matrix
A <- matrix(x,5,10)

Let’s make name our rows and columns

#name and assignments vectors
names <- c("Sara", "Jill", "Jared", "Kim", "Don")
assignments <- c("Quiz 1", "Quiz 2", "Test 1", "Quiz 3", "Quiz 4", "Midterm", "Quiz 5", "Quiz 6", "Test 2","Final")
#assign labels to columns and rows
row.names(A) <- names
colnames(A) <- assignments

Let’s graph it

Start simple


Note our X axis goes to 5 – we are graphing students against grades.

What if I want to graph assignments against grades? Simple, transpose the matrix


Okay, but the numbers are distracting. Let’s do something about that.

matplot(t(A), type="b")

I don’t like the numbers

# replace numbers with shapes
matplot(t(A), type="b", pch=15:19)

pch 15:19 lets us rotate through 4 shapes (15 – 19) – try different numbers on your own

I am adding a color element next. You will see the need for this in our next section.

 matplot(t(A), type="b", pch=15:18, col=c(1:5))

This has no current effect on our graph


Let’s add a legend

legend("bottomleft", inset=0.01, legend=names, col=c(1:5),pch=15:19,
bg= ("white"), horiz=F)

okay, here is the syntax: legend(location, padding, legend data, colors, symbols, background, horizontal vs vertical layout)

This is why I added the color element to our matplot, so the colors in the legend would match up to the colors in my graph.




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