Graph annotation with ggplot

To produce a publication-quality data visualization, we often need to annotate the graph with various texts, symbols, or even shapes. In this recipe, we will learn how we can easily annotate an existing graph.

Getting ready

In this recipe, we will use the disA and disD variables from ggplotdata. Let's call ggplotdata for this recipe. We also need to call the grid and gridExtra libraries for this recipe.

How to do it...

In this recipe, we will execute the following annotation on an existing scatter plot. So, the whole procedure will be as follows:

  1. Create a scatter plot.
  2. Add customized text within the plot.
  3. Highlight a certain region to indicate extreme values.
  4. Draw a line segment with an arrow within the scatter plot to indicate a single extreme observation.

Now, we will implement each of the steps one by one:

library(grid)
library(gridExtra)
# creating scatter plot and print it
annotation_obj <- ggplot(data=ggplotdata,aes(x=disA,y=disD))+geom_point()
annotation_obj

# Adding custom text at (18,29) position
annotation_obj1 <- annotation_obj + annotate(geom="text",x=18,y=29,label="Extreme value",size=3)
annotation_obj1

# Highlight certain regions with a box 
annotation_obj2 <- annotation_obj1+ 
annotate("rect", xmin = 24, xmax = 27,ymin=17,ymax=22,alpha = .2)
annotation_obj2

# Drawing line segment with arrow 
annotation_obj3 <- annotation_obj2+
annotate("segment",x = 16,xend=17.5,y=25,yend=27.5,colour="red", arrow = arrow(length = unit(0.5, "cm")),size=2)
annotation_obj3

The preceding four steps are displayed in the following single graph:

How to do it...

How it works...

The annotate() function takes input of a geom such as "segment" and "text", and then it takes another input regarding position of that geom that is where to draw or where to place.. In this particular recipe, we used three geom instances, such as text to write customized text within the plot, rect to highlight a certain region in the plot, and segment to draw an arrow. The alpha argument represents the transparency of the region and size argument to represent the size of the text and line width of the line segment.

How it works...
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