Multiple layers with ggplot2

STAT1003 – Statistical Techniques

Dr. Emi Tanaka

Australian National University

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Layering plots

You can add more than one layer.
The order of layer matters.

A layer inherits the data and mapping from the initialised ggplot object by default

Layer-specific data and aesthetic mapping

For each layer, aesthetic and/or data can be overwritten.

Case study 🚜 Iowa farmland values by county

Drawing maps

Drawing maps require spatial/boundary data that defines the shapes of the regions.

Layer specific aesthetic

Layer specific aesthetic are not inherited by other layers.

Layer specific data

Layer specific data can overwrite the inherited data.

Layer specific data as a function of inherited data

Annotation layer

annotate() allows you to add elements to plots without a data.frame

More features of `ggplot2`

theme() to modify non-data components of the plot
facet_wrap() and facet_grid() for small multiples
scale_*() to modify scales
guides() to modify legends
labs() to modify labels and titles
ggsave() to save plots to files
… etc.

`ggplot2` extensions

https://www.ggplot2-exts.org/

Featuring: `ggincerta`

Your tutor, Maggie Ma, developed an extension package ggincerta to visualise uncertainty in ggplot2 as part of her PhD work!

library(ggincerta)
ggplot(nc) + 
  aes(fill = duo(value, sd)) +
  geom_sf()

Summary

You can construct plots with multiple layers in ggplot2.
The order of the layer matters.

A layer inherits the data and mapping from the initialised ggplot object by default
But the data and mappings for each layer can be overwritten.
There are many more features and extensions of ggplot2 to explore!