Understanding the Legend in R Core: A Deep Dive into Horizontal Boxes and Labels

R core’s legend() function is a powerful tool for creating horizontal boxes with associated labels. However, there are certain limitations and quirks to this function that can affect its appearance on different devices. In this article, we’ll delve into the world of R core’s legend function, exploring why device dimensions matter and how to overcome the truncation issue.

What is the Legend Function in R Core?

The legend() function in R core is used to create a sequence of horizontal boxes with associated labels. These boxes can be filled with different colors and are typically placed at the bottom of a plot or chart. The legend function takes several parameters, including xpd, horiz, bty, cex, and others, which we’ll explore in more detail below.

The Issue: Truncation on Smaller Devices

When using the legend() function to create a sequence of horizontal boxes with associated labels, there’s an issue with truncation on smaller devices. As explained in the original Stack Overflow post, when a new device or window is first created, only two innermost elements (i.e., 2 filled rectangles and associated labels) of the legend are rendered, while the left and right elements are truncated from the device view.

This truncation issue can be observed on various devices, including desktops, laptops, tablets, and smartphones. The problem arises because R core’s legend() function doesn’t always adapt to the screen size or device resolution.

Exploring the Parameters of the Legend Function

To understand why device dimensions matter and how to overcome the truncation issue, let’s explore some of the key parameters of the legend() function:

xpd: This parameter specifies whether the legend boxes should extend into the plot area or stay within the margins. When set to NA, as in our example code, it tells R core to render the legend boxes only in the outer margins of the device view.
horiz: This parameter determines the orientation of the legend boxes. Set to TRUE, it creates horizontal boxes with labels, while setting it to FALSE would create vertical boxes with labels.
bty: The bty parameter specifies the type of box that should be used for the legend. It can take values like "n", "l", or "i". When set to "n", it creates a null box, which is useful when no box needs to be displayed.
cex: This parameter scales the size of the font used in the labels and boxes. It’s essential for adjusting the appearance of the legend.
text.width: As we’ll see later, this parameter allows you to control the allocated width for each label in the legend.

Adjusting the Legend Function for Compactting or Expanding Allocated Width

To overcome the truncation issue and create a more flexible legend function, we can adjust the parameters mentioned above. One of the most effective adjustments is using the text.width parameter.

The text.width parameter allows you to specify the allocated width for each label in the legend. By setting it to a fixed value or a range of values, you can control how much space is allocated for each label.

In our example code, we use the text.width parameter to allocate an equal swath of device space along the horizontal direction of the plot for each element (i.e., filled rectangle with label). We achieve this by setting it to rep(meanLabelLen/7.5, length(legendLabels)), which calculates a width based on the mean length of the labels and divides it into equal parts.

Additional Horizontal Adjustments

While adjusting the text.width parameter is an effective way to control the allocated width for each label, there are other horizontal adjustments you can make to further customize your legend function:

adj: This parameter adjusts the x-position of the labels relative to the corresponding boxes. It takes a value between 0 and 1, where 0 means the labels will be aligned with the left edge of the box, and 1 means they’ll be aligned with the right edge.
inset: The inset parameter adjusts the x-position of the legend relative to the plot area. It takes a value in the form of c(x1, x2), where x1 is the starting point and x2 is the ending point.

Conclusion

R core’s legend() function provides an effective way to create horizontal boxes with associated labels. However, its limitations and quirks can affect its appearance on different devices. By understanding the parameters of this function, including text.width, and adjusting them accordingly, you can create a more flexible and responsive legend function that adapts to various screen sizes and device resolutions.

Additionally, experimenting with other horizontal adjustments like adj and inset can help fine-tune your legend’s appearance. Remember to test your code on different devices to ensure the best results for your specific use case.

Additional Resources

For further learning about R core’s legend() function, we recommend checking out related articles or online documentation resources:

By exploring these additional resources and experimenting with different adjustments, you’ll become more proficient in using the legend() function to create visually appealing and responsive legends for your R projects.

Last modified on 2024-07-22