Using Distance Between Y-Axis Ticks as Variable in ggplot2

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In this article, we will explore how to use the distance between the ticks on the y-axis as a variable within ggplot functions. This can be particularly useful when creating annotations that need to be responsive to the scale of the graph.

Introduction

The ggplot package in R provides an easy-to-use interface for creating high-quality, informative graphics. One of its key features is the ability to customize various aspects of the plot, including the layout and appearance of the axes. In this article, we will focus on using the distance between y-axis ticks as a variable within ggplot functions.

Background

In ggplot, the y-axis tick distances are determined by the panel.ranges argument in the ggplot_build() function. This function takes a ggplot object and returns an object that contains information about the plot, including the panel ranges.

The distance between two consecutive y-axis ticks is calculated as follows:

The first tick is obtained from the y.minor_source element of the panel.ranges array.
The second tick is subtracted from the first tick to obtain the distance between them.

This approach allows us to extract the distance between y-axis ticks and use it as a variable in our plot.

Example Data

For this example, we will use the following dataset:

EXse2 <- data.frame(wk=c(1,2), EX=c(4.457143, 2.580952), se=c(0.4209481,0.5519333))

Step-by-Step Solution

To solve this problem, we will follow these steps:

Step 1: Create the Plot

First, we need to create a plot using ggplot and define the aesthetics for the x and y variables.

p1 <- 
  ggplot(EXse2, aes(x=factor(wk), y=EX, group=1))+
  geom_errorbar(aes(ymin=EX-se, ymax=EX+se), width=.2, size=1)+
  geom_line(size=1)+
  geom_point(size=2)+
  xlab("period")+
  ylab(expression(bar(EX)))+
  theme(
    panel.background=element_blank(),
    panel.border = element_blank(), 
    panel.grid.minor.x=element_blank(),
    panel.grid.major.x=element_blank(),
    panel.grid.minor.y = element_line(colour = "#d9d9d9"), 
    panel.grid.major.y = element_line(colour = "#bfbfbf"),  
    axis.line = element_line()
  )

Step 2: Extract Axis Limits

Next, we need to extract the axis limits from the plot using ggplot_build().

# extract axis limits
yminticks <- ggplot_build(p1)$panel$ranges[[1]]$y.minor_source

Step 3: Calculate Distance Between Ticks

We can now calculate the distance between two consecutive y-axis ticks by subtracting the first tick from the second.

# subtract first tick from second
var1 <- yminticks[2] - yminticks[1]

Step 4: Add Annotation

Finally, we can add an annotation to the plot using annotate() and specify the distance between ticks as a variable.

p1 + 
  annotate("text", label=labEX, x=1.5, y=(mean(EXse2$EX[1:2])+var1))

Conclusion

In this article, we have explored how to use the distance between y-axis ticks as a variable within ggplot functions. We covered the following steps:

Creating a plot using ggplot
Extracting axis limits from the plot using ggplot_build()
Calculating the distance between two consecutive y-axis ticks
Adding an annotation to the plot and specifying the distance between ticks as a variable

By following these steps, you can create high-quality plots with responsive annotations that are sensitive to the scale of the graph.

Example Use Cases

Here is an example use case for this technique:

Suppose we want to create a scatterplot where the x-axis represents time and the y-axis represents temperature. We want to add annotations to indicate the average temperature at each point in time, where the size of the annotation is proportional to the uncertainty in the measurement.

We can achieve this by using the distance between y-axis ticks as a variable within ggplot().

p2 <- 
  ggplot(data.frame(time=c(1,2), temp=c(20,25), se=c(0.5,0.8)), aes(x=time, y=temp))+
  geom_point()+
  annotate("text", label=paste("Temp:", round(mean(temp),2)), x=1.5, y=(mean(temp)+var1))

In this example, the distance between ticks (var1) is calculated as follows:

# calculate distance between ticks
yminticks <- ggplot_build(p2)$panel$ranges[[1]]$y.minor_source
var1 <- yminticks[2] - yminticks[1]

The annotation is then added to the plot using annotate(), where the size of the label is proportional to the uncertainty in the measurement.

p2 + 
  annotate("text", label=paste("Temp:", round(mean(temp),2)), x=1.5, y=(mean(temp)+var1))

By using the distance between y-axis ticks as a variable within ggplot(), we can create high-quality plots with responsive annotations that are sensitive to the scale of the graph.

Last modified on 2023-10-16