A Comprehensive Guide to Plotting Legends Outside the Plotting Area in Base Graphics

Challenges of Legend Positioning in Base Graphics

In R's base graphics system, legends are typically confined within the plotting area by default. This limitation poses challenges in various data visualization scenarios, particularly when dealing with extensive legend content or when maintaining a clean plotting area is essential. The traditional legend() function, under normal circumstances, clips legend elements to the plot boundaries, restricting layout flexibility.

Core Solution: The par(xpd=TRUE) Parameter

The key to extending legends beyond the plotting area lies in understanding R's graphical device clipping mechanism. By setting par(xpd=TRUE), the default clipping behavior is disabled, allowing graphical elements to extend outside the plotting region. This parameter controls the display range of graphical elements; when set to TRUE, graphics can be drawn across the entire device area.

Here's a complete example demonstrating how to use par(xpd=TRUE) to position a legend to the right of the plotting area:

# Set random seed for reproducible results
set.seed(1)

# By default, clipping is enabled
par(xpd=FALSE)

# Create basic scatter plot
plot(1:3, rnorm(3), pch=1, lty=1, type="o", ylim=c(-2,2), bty='L')
lines(1:3, rnorm(3), pch=2, lty=2, type="o")

# This legend will be clipped to the plotting area
legend(2.8, 0, c("Group A", "Group B"), pch=c(1,2), lty=c(1,2))

# Disable clipping
par(xpd=TRUE)

# Now the legend can be placed outside the plotting area
legend(2.8, -1, c("Group A", "Group B"), pch=c(1,2), lty=c(1,2))

Parameter Adjustment and Precise Positioning

In practical applications, legend positioning requires careful adjustment based on the specific graphic. The coordinate parameters in the legend() function determine the exact placement of the legend. With par(xpd=TRUE) enabled, these coordinates can extend beyond the original plotting region boundaries. It's important to note that the coordinate system remains based on the original plotting area, requiring reasonable estimation of coordinates beyond the boundaries.

For improved visual results, consider combining with par(mar) parameter to adjust margin settings:

# Increase right margin to create space for legend
par(mar=c(5.1, 4.1, 4.1, 8.1), xpd=TRUE)

# Create the plot
plot(1:3, rnorm(3), pch=1, lty=1, type="o", ylim=c(-2,2))
lines(1:3, rnorm(3), pch=2, lty=2, type="o")

# Position legend in the reserved right space
legend(3.2, 0, c("Group A", "Group B"), pch=c(1,2), lty=c(1,2))

Comparison of Alternative Approaches

Beyond using par(xpd=TRUE), several other methods exist for external legend placement:

Using Negative Inset Values

Setting the inset parameter to negative values pushes the legend away from the plotting area:

par(mar=c(5.1, 4.1, 4.1, 8.1), xpd=TRUE)
plot(y ~ x, A, ylim=range(c(A$y, B$y)), xlim=range(c(A$x, B$x)), pch=1)
points(y ~ x, B, pch=3)
legend("topright", inset=c(-0.2,0), legend=c("A","B"), pch=c(1,3))

Layout Function Approach

Using the layout() function to create complex graphic layouts with dedicated legend areas:

# Create 2-column layout with narrow right column for legend
layout(matrix(c(1,2), ncol=2), widths=c(3,1))

# Main plotting area
plot(1:3, rnorm(3), pch=1, lty=1, type="o", ylim=c(-2,2))
lines(1:3, rnorm(3), pch=2, lty=2, type="o")

# Legend area
plot.new()
legend("center", c("Group A", "Group B"), pch=c(1,2), lty=c(1,2))

Practical Implementation Recommendations

When selecting an external legend placement method, consider the following factors:

Graphic Complexity: For simple graphics, par(xpd=TRUE) combined with coordinate adjustment provides the most straightforward approach. For complex multi-panel graphics, layout functions may be more appropriate.

Reproducibility: If consistent legend positioning across multiple graphics is required, consider encapsulating legend settings in functions to ensure parameter consistency.

Device Compatibility: Different graphic devices (PNG, PDF, screen display) may handle out-of-bound graphical elements differently, requiring thorough testing.

Advanced Techniques and Considerations

Several technical details warrant attention when implementing external legends:

Coordinate System Understanding: R's base graphics system uses normalized device coordinates (NDC); understanding this coordinate system facilitates precise legend positioning.

Graphical Parameter Management: After modifying graphical parameters with par(), restore original settings after plotting to avoid affecting subsequent graphics:

# Save original parameters
old_par <- par(no.readonly=TRUE)

# Modify parameters for plotting
par(xpd=TRUE, mar=c(5,4,4,6))
plot(1:10, 1:10)
legend(11, 5, "External Legend", pch=1)

# Restore original parameters
par(old_par)

Legend Style Control: Various parameters in the legend() function enable fine-grained control over legend appearance, including font size, colors, borders, ensuring consistency with overall graphic style.

Conclusion

The par(xpd=TRUE) parameter provides flexible legend positioning capabilities within R's base graphics system. This approach proves both simple and effective, meeting the requirements of most data visualization tasks. Combined with margin adjustments and precise coordinate positioning, it enables the creation of aesthetically pleasing and functionally sound statistical graphics. For more complex layout needs, layout functions or custom legend functions offer enhanced control capabilities.