Chapter 3 Scatter Plot

Scatter plots can help you identify the relationship between two data samples. A scatter plot is a simple plot of one variable against another.

3.1 How to draw a scatter plot in base R?

Let’s consider mtcars data set being available in R.

head(mtcars) #the first six observation of mtcars being an available in R is shown

##                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
## Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
## Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
## Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
## Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
## Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
## Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

plot(mtcars$wt, mtcars$mpg) #to draw a scatter plot, use plot command 

3.2 How can we draw scatter plot in ggplot2?

qplot(mtcars$wt, mtcars$mpg) #first way

If the two vectors are already in the same data frame, you can use the following syntax:

qplot(wt, mpg, data=mtcars) 

Another way

ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point() #suggested way

#geom_point function creates your plot as scatter plot

This way is the better one.

3.3 Adding Label Names and Titles

To add title and label names into your plot, labs command is used. Consider the previous plot.

ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()+labs(title="Scatter plot of wt and mpg",x="wt", y = "mpg") 

#title is used to add title
#x change the name of x axes
#y change the name of y axes

The labs command can be used not only scatter plot, but also other plots to add title etc.

3.4 Drawing scatter plot of two continious variables conditioned on one categorical variables

head(mtcars)

##                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
## Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
## Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
## Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
## Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
## Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
## Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

table(mtcars$cyl) #produces frequency table

## 
##  4  6  8 
## 11  7 14

ggplot(data = mtcars,aes(x = mpg,y = disp,colour = factor(cyl))) + geom_point()+labs(title="Scatter plot of wt and mpg with respect to cylinder",x="wt", y = "mpg") 

3.5 Changing color scale in legend

c <- ggplot(mtcars,aes(x = mpg, y = disp, color = cyl))  + geom_point()
c + scale_color_gradient2(low = "yellow", high = "red")

When you use scale_color_gradient2 command, do not use factor() for coloring your plot.

3.6 Drawing multiple scatter plot

By using facet_wrap command, you can easily produce multiple scatter plot.

ggplot(data = mtcars,aes(x = mpg,y = disp)) + geom_point()+labs(title="Scatter plot of wt and mpg with respect to cylinder",x="wt", y = "mpg")+facet_wrap("cyl")