When creating visualizations in R, one common task is to include a legend that helps interpret the data represented in the plot. However, in many cases, the default location of the legend can interfere with the visualization itself, obscuring important details. In this article, we will discuss how to plot a legend outside of the plotting area in base graphics.
Understanding the Problem
The problem at hand is that while plotting graphs using base R graphics, the default behavior places the legend within the plotting area, which can lead to overlapping elements or cluttered visuals. To create cleaner and more interpretable plots, we want to position the legend outside of the main plotting area.
Example Scenario
Let’s say you are working with a simple dataset containing two variables that you wish to plot. Here is the original code that generates a basic scatter plot with a legend positioned within the plotting area:
# Sample data
x <- 1:10
y1 <- x + rnorm(10)
y2 <- x + rnorm(10, sd=1.5)
# Basic plot with a legend
plot(x, y1, col="blue", pch=19, xlab="X-axis", ylab="Y-axis", main="Plot with Legend Inside")
points(x, y2, col="red", pch=19)
legend("topright", legend=c("Series 1", "Series 2"), col=c("blue", "red"), pch=19)
In the code above, the legend is positioned in the top-right corner of the plot, which can sometimes obscure part of the plotted points.
Moving the Legend Outside the Plotting Area
To achieve a clearer visualization, we can position the legend outside of the plotting area. This can be done using the xpd parameter and adjusting the legend function's x and y coordinates. Here’s how to do it:
# Basic plot
plot(x, y1, col="blue", pch=19, xlab="X-axis", ylab="Y-axis", main="Plot with Legend Outside")
points(x, y2, col="red", pch=19)
# Add a legend outside the plotting area
legend("topright", legend=c("Series 1", "Series 2"), col=c("blue", "red"), pch=19, xpd=TRUE)
However, to effectively position the legend outside the plot, we might want to utilize coordinates relative to the plot margins. For example, we can use x = "n" and y = par("usr")[4] + 0.5 to position it based on the upper limit of the plotting area.
Here’s an improved version of our previous plot code:
# Basic plot
plot(x, y1, col="blue", pch=19, xlab="X-axis", ylab="Y-axis", main="Plot with Legend Outside")
points(x, y2, col="red", pch=19)
# Calculate plot limits and add legend
legend("topright", legend=c("Series 1", "Series 2"), col=c("blue", "red"), pch=19, xpd=TRUE, inset=c(0.1, 0.05))
Insights and Considerations
-
Inserting the Legend: The
insetparameter allows you to control how far the legend is placed from the corners of the plot, providing flexibility to prevent overlap with the data points. -
Using
xpd = TRUE: Thexpdparameter enables plotting beyond the normal plot region. When this is set toTRUE, the legend can be placed outside of the boundaries defined by the plotting area. -
Customizing the Legend: You can also customize the legend’s appearance by adjusting the
cexparameter to change text size orbtyto modify the box type.
Conclusion
Positioning a legend outside of the plotting area in base graphics can significantly enhance the clarity of your visualizations. By employing the xpd parameter and adjusting the position using coordinates or the inset parameter, you can create cleaner and more effective plots.
By mastering these techniques, you not only improve the aesthetic quality of your visualizations but also enhance the communicative power of your data presentations.
Additional Resources
By applying these techniques, you can elevate your data visualization skills and ensure your graphics convey the intended message effectively.
