How to Plot a Time Series in R

Plotting time series data is an essential part of data analysis and interpretation. Visualizing the data provides valuable insights into underlying trends, seasonal patterns, and potential anomalies. In R, there are numerous libraries and built-in functions that allow for sophisticated and informative time series plots. This article will serve as a comprehensive guide on how to plot time series in R, covering a range of methods from basic to advanced.

Table of Contents

1. Getting Started with R and Time Series
2. Basic Plotting with Base R
3. Enhancing Base R Plots
4. Advanced Plotting with ggplot2
5. Time Series Decomposition Plots
6. Specialized Time Series Plots
7. Best Practices for Time Series Plotting

1. Getting Started with R and Time Series

For the purposes of this guide, we will use the built-in AirPassengers dataset, which contains monthly totals of international airline passengers from 1949 to 1960.

Load the dataset into your R environment:

data("AirPassengers")

2. Basic Plotting with Base R

The simplest way to plot time series data in R is by using the plot() function from base R. Here’s how you can do it:

plot(AirPassengers, type = "l")

In this code, type = "l" indicates that we want a line plot.

3. Enhancing Base R Plots

While the basic plot is useful, you may want to add additional features to make the plot more informative.

Adding Labels and Titles

You can add an X-axis label, Y-axis label, and a title using the xlab, ylab, and main arguments, respectively.

plot(AirPassengers, type = "l",
     xlab = "Year",
     ylab = "Number of Passengers",
     main = "Airline Passenger Data from 1949 to 1960")

Adding Grid Lines

To add grid lines, you can use the abline() function:

plot(AirPassengers, type = "l")
abline(h = seq(100, 700, by = 100), col = "gray", lty = "dotted")

4. Advanced Plotting with ggplot2

For more advanced and aesthetic plots, you can use the ggplot2 package. First, install and load the package:

install.packages("ggplot2")
library(ggplot2)

Basic ggplot2

First, convert the AirPassengers data into a data frame:

install.packages("zoo")
library(zoo)

df <- data.frame(
  Month = as.Date(as.yearmon(time(AirPassengers))),
  Passengers = coredata(AirPassengers)
)

In this example, I used the as.Date and as.yearmon functions from the zoo package to convert the time index of the AirPassengers data into date format.

Now, you can create the ggplot:

ggplot(data = df, aes(x = Month, y = Passengers)) +
  geom_line() +
  xlab("Year") +
  ylab("Number of Passengers") +
  ggtitle("Airline Passenger Data from 1949 to 1960")

Adding Layers

With ggplot2, you can easily add layers to your plot. For example, you can add a smoothed line using geom_smooth():

# Create the ggplot
ggplot(data = df, aes(x = Month, y = Passengers)) +
  geom_line() +
  geom_smooth(method = 'loess') +  # Adding a smoothed line
  xlab("Year") +
  ylab("Number of Passengers") +
  ggtitle("Airline Passenger Data with Smoothed Line")

5. Time Series Decomposition Plots

Decomposing a time series can provide insights into its structure. The stl() function can be used for this. You can then plot the decomposed object:

# Decompose the time series
decomposed <- stl(AirPassengers, s.window = "periodic")

# Plot the decomposed time series
plot(decomposed)

6. Specialized Time Series Plots

Packages like forecast and xts offer specialized plots for time series data.

Forecast Package

Install and load the package:

install.packages("forecast")
library(forecast)

Now you can use the autoplot() function:

autoplot(AirPassengers)

XTS Package

For financial time series, xts provides powerful plotting methods:

install.packages("xts")
library(xts)

# Create an xts object and plot
air_xts <- as.xts(AirPassengers)
plot.xts(air_xts, main = "XTS Plot")

7. Best Practices for Time Series Plotting

• Always label your axes.
• Use appropriate scales.
• Include legends for multiple series.
• Pay attention to color and line types for readability.

Conclusion

R offers a multitude of ways to visualize time series data, ranging from simple plots in base R to more sophisticated and interactive plots using packages like ggplot2. Understanding how to leverage these tools can provide invaluable insights into your time series data. This comprehensive guide should equip you with the techniques and best practices you need to effectively visualize time series data in R.