# Scatterplots

## Simple Scatterplot

There are many ways to create a scatterplot in R. The basic function is plot(x, y), where x and y are numeric vectors denoting the (x,y) points to plot.

```# Simple Scatterplot attach(mtcars) plot(wt, mpg, main="Scatterplot Example",    xlab="Car Weight ", ylab="Miles Per Gallon ", pch=19) ```

(To practice making a simple scatterplot, try this interactive example from DataCamp.)

```# Add fit lines abline(lm(mpg~wt), col="red") # regression line (y~x) lines(lowess(wt,mpg), col="blue") # lowess line (x,y) ```

The scatterplot( ) function in the car package offers many enhanced features, including fit lines, marginal box plots, conditioning on a factor, and interactive point identification. Each of these features is optional.

```# Enhanced Scatterplot of MPG vs. Weight # by Number of Car Cylinders library(car) scatterplot(mpg ~ wt | cyl, data=mtcars,    xlab="Weight of Car", ylab="Miles Per Gallon",    main="Enhanced Scatter Plot",    labels=row.names(mtcars)) ```

## Scatterplot Matrices

There are at least 4 useful functions for creating scatterplot matrices. Analysts must love scatterplot matrices!

```# Basic Scatterplot Matrix pairs(~mpg+disp+drat+wt,data=mtcars,    main="Simple Scatterplot Matrix")```

The lattice package provides options to condition the scatterplot matrix on a factor.

```# Scatterplot Matrices from the lattice Package library(lattice) splom(mtcars[c(1,3,5,6)], groups=cyl, data=mtcars,    panel=panel.superpose,    key=list(title="Three Cylinder Options",    columns=3,    points=list(pch=super.sym\$pch[1:3],    col=super.sym\$col[1:3]),    text=list(c("4 Cylinder","6 Cylinder","8 Cylinder")))) ```

The car package can condition the scatterplot matrix on a factor, and optionally include lowess and linear best fit lines, and boxplot, densities, or histograms in the principal diagonal, as well as rug plots in the margins of the cells.

```# Scatterplot Matrices from the car Package library(car) scatterplot.matrix(~mpg+disp+drat+wt|cyl, data=mtcars,    main="Three Cylinder Options")```

The gclus package provides options to rearrange the variables so that those with higher correlations are closer to the principal diagonal. It can also color code the cells to reflect the size of the correlations.

```# Scatterplot Matrices from the glus Package library(gclus) dta <- mtcars[c(1,3,5,6)] # get data dta.r <- abs(cor(dta)) # get correlations dta.col <- dmat.color(dta.r) # get colors # reorder variables so those with highest correlation # are closest to the diagonal dta.o <- order.single(dta.r) cpairs(dta, dta.o, panel.colors=dta.col, gap=.5, main="Variables Ordered and Colored by Correlation" ) ```

## High Density Scatterplots

When there are many data points and significant overlap, scatterplots become less useful. There are several approaches that be used when this occurs. The hexbin(x, y) function in the hexbin package provides bivariate binning into hexagonal cells (it looks better than it sounds).

```# High Density Scatterplot with Binning library(hexbin) x <- rnorm(1000) y <- rnorm(1000) bin<-hexbin(x, y, xbins=50) plot(bin, main="Hexagonal Binning") ``` click to view

Another option for a scatterplot with significant point overlap is the sunflowerplot. See help(sunflowerplot) for details.

Finally, you can save the scatterplot in PDF format and use color transparency to allow points that overlap to show through (this idea comes from B.S. Everrit in HSAUR).

```# High Density Scatterplot with Color Transparency pdf("c:/scatterplot.pdf") x <- rnorm(1000) y <- rnorm(1000) plot(x,y, main="PDF Scatterplot Example", col=rgb(0,100,0,50,maxColorValue=255), pch=16) dev.off()```

Note: You can use the col2rgb( ) function to get the rbg values for R colors. For example, col2rgb("darkgreen") yeilds r=0, g=100, b=0. Then add the alpha transparency level as the 4th number in the color vector. A value of zero means fully transparent. See help(rgb) for more information.

## 3D Scatterplots

You can create a 3D scatterplot with the scatterplot3d package. Use the function scatterplot3d(x, y, z).

```# 3D Scatterplot library(scatterplot3d) attach(mtcars) scatterplot3d(wt,disp,mpg, main="3D Scatterplot")``` click to view

```# 3D Scatterplot with Coloring and Vertical Drop Lines library(scatterplot3d) attach(mtcars) scatterplot3d(wt,disp,mpg, pch=16, highlight.3d=TRUE,   type="h", main="3D Scatterplot") ``` click to view

```# 3D Scatterplot with Coloring and Vertical Lines # and Regression Plane library(scatterplot3d) attach(mtcars) s3d <-scatterplot3d(wt,disp,mpg, pch=16, highlight.3d=TRUE,   type="h", main="3D Scatterplot") fit <- lm(mpg ~ wt+disp) s3d\$plane3d(fit)``` click to view

### Spinning 3D Scatterplots

You can also create an interactive 3D scatterplot using the plot3D(x, y, z) function in the rgl package. It creates a spinning 3D scatterplot that can be rotated with the mouse. The first three arguments are the x, y, and z numeric vectors representing points. col= and size= control the color and size of the points respectively.

```# Spinning 3d Scatterplot library(rgl) plot3d(wt, disp, mpg, col="red", size=3) ``` click to view

You can perform a similar function with the scatter3d(x, y, z) in the Rcmdr package.

```# Another Spinning 3d Scatterplot library(Rcmdr) attach(mtcars) scatter3d(wt, disp, mpg) ```

## To Practice

Try the creating scatterplot exercises in this course on data visualization in R.