Aligning Shapes in ggplot Legends with Custom Shapes: A Step-by-Step Guide

Understanding ggplot Shape and Legend Alignment

In this article, we will delve into the world ofggplot2, a powerful data visualization library in R. We will explore how to align shapes in a legend with their corresponding data points in a plot.

Introduction to ggplot

ggplot2 is a system for creating beautiful graphics. It is built on top of the base graphics package and provides a high-level interface for data visualization. The name “ggplot” comes from the phrase “grammar of graphics.” It follows the grammar of graphics, which consists of three main components: layers, aesthetics, and facets.

Layers

In ggplot2, a layer is a single element in a plot that can be added on top of other elements using the + operator. Common types of layers include geometric shapes, statistical transformations, and facetting.

Aesthetics

An aesthetic is a mapping between data values and graphical properties. In ggplot2, aesthetics are used to specify which columns of the dataset should be mapped to which visual properties (such as color, size, and shape). The most common aesthetics in ggplot2 include color, size, and shape.

Faceting

Facets are separate subplots that can be added to a plot using the facet() function. Each facet contains its own layer or layers.

The Problem with Custom Shapes

In this article, we will explore how to align shapes in a legend with their corresponding data points in a plot when custom shapes are used.

The question posed by the user is as follows:

“If you wish to have a custom legend rather than reshaping your data (which would be the standard way to use ggplot), you need to map the shape as an aesthetic.”

This means that we must specify the shape of each point in our plot using the shape aesthetic and give it the same name that will appear on the legend.

Mapping Shape as an Aesthetic

In order for shapes to be aligned with their corresponding data points, the shape must be mapped as an aesthetic. This can be done by placing the shape = </code> inside the aes() call:

geom_point(aes(y = wt, color = "wt name", shape = "wt name"), size = 4)

However, this will not align with other shapes on the same plot that do not have a specified shape. To resolve this issue, we must ensure that all scales are given the same name = </code> if we want them to be merged in the legend:

scale_shape_manual(values = c("wt name" = 19, "drat name" = 8 ), 
                    name = "legend")

Example Code

Here is an example code snippet that demonstrates how to map shape as an aesthetic and align it with other shapes on the same plot:

library(ggplot2)

mtcars$time &lt;- 1:nrow(mtcars)

ggplot(mtcars, aes(x = time)) +
  
  #wt
  geom_line(aes(y = wt, color = "wt name"), size = 1.5) +
  geom_point(aes(y = wt, color = "wt name", shape = "wt name"), size = 4) +
  
  #drat
  geom_line(aes(y = drat   , color = "drat name"), size = 1.5) +
  geom_point(aes(y = drat   , color = "drat name", shape ="drat name"), size = 4) +
  
  scale_colour_manual(values = c("wt name" = "#F8766D",  
                                "drat name"   = "#B79F00"),
                      name = "legend") +
  scale_shape_manual(values = c("wt name" = 19, "drat name" = 8 ), 
                     name = "legend")

Conclusion

In conclusion, in order to align shapes in a legend with their corresponding data points when custom shapes are used, we must map the shape as an aesthetic. We also must ensure that all scales are given the same name = </code> if we want them to be merged in the legend.

Additionally, using scale_shape_manual will align shapes in the legend only for that specific scale (color). If there is a need for multiple legends but a single color and shape in each, you can use scale_size_manual, scale_colour_manual with breaks argument, and guides() to hack the breaks and guide.

We have explored the basics of ggplot2 and how to align shapes in a legend using custom shapes.

Last modified on 2023-08-30