ggplot Legend for Second Axis

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In this article, we will explore a common issue when creating plots with ggplot2 in R: mixing multiple aesthetic mappings on the same axis. Specifically, we’ll examine how to create a legend for a second axis that shows cumulative values without overlapping with other elements.

Introduction

ggplot2 is a powerful data visualization library for R that provides a consistent and effective way to create high-quality plots. However, when working with multiple axes or complex aesthetic mappings, it can be challenging to achieve the desired layout and clarity in the legend. In this article, we’ll explore a solution to create a separate legend for a second axis that displays cumulative values without overlapping with other elements.

Background

Before we dive into the solution, let’s briefly review some key concepts:

• Aesthetics: Aesthetic mappings are used to map variables from the data to visual properties of the plot. For example, aes(x = Datum, y = mittelwert) maps the Datum variable to the x-axis and the mittelwert variable to the y-axis.
• Geoms: Geometric primitives (geoms) are used to define the shapes of the plots. Common geoms include geom_point(), geom_bar(), and geom_line().
• Scales: Scales are used to transform variables from the data into numerical values that can be displayed on the plot. For example, scale_y_continuous() creates a continuous scale for the y-axis.

The Problem

In your original code, you’re using ggplot() with multiple aesthetic mappings on the same axis:

geom_point(aes(Datum, kumsum_1189), shape =18, color = "blue") +
scale_y_continuous(name = "Bodenfeuchte in %", sec.axis = sec_axis(~., name = "Kummulative Regenmenge 10 Tage"))

This can cause the legend for kumsum_1189 to overlap with the legend for Transtyp.

The Solution

To create a separate legend for the second axis, we need to use the sec.axis() function within scale_y_continuous(). Here’s an updated code snippet that demonstrates how to achieve this:

BF_1189 <- BF_1189 %>% 
  mutate(Regen = "Kummulative")

for (i in solle_1189) {
  BF_1189 %>% filter(Soll == i) %>% 
    group_by(Datum, Soll, Transtyp) %>% 
    ggplot(aes(x = Datum, y = mittelwert))+
    geom_bar(aes(fill = Transtyp), stat = "Identity",
             position = "dodge") + 
    geom_point(aes(y = kumsum_1189, size = Regen),
               shape = 18, color = "blue") +
    scale_y_continuous(name = "Bodenfeuchte in %", sec.axis = sec_axis(~., name = "Kummulative Regenmenge 10 Tage")) +
    scale_x_date(breaks = sdates)+
    theme_minimal()+
    theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust=1),
          axis.text = element_text(size = 9),
          axis.title = element_text(size = 10, face = "bold")) +
    scale_fill_grey() +
    labs(title = paste0("Soll ", i)) -> pb
  print(pb)
  #ggsave(paste0("Z:/FOR/FOR-Projects/IPP_Soelle/Daten/Data_Raw/Max_2021/Grafen/Bodenfeuchte_fdr_kumnied_", i, ".png"))
}

By using sec_axis() within scale_y_continuous(), we can create a separate legend for the second axis ("Kummulative Regenmenge 10 Tage"). The size = Regen aesthetic mapping allows us to control the size of the points on the plot.

Conclusion

In this article, we explored how to create a separate legend for a second axis in ggplot2. By using the sec_axis() function within scale_y_continuous(), we can achieve clear and distinct legends without overlapping with other elements. This technique is particularly useful when working with multiple axes or complex aesthetic mappings.

Additional Resources

• ggplot2 documentation
• sec_axis() function
• [scale_y_continuous() function](https://ggplot2.tidyverse.org/functions scales_y_continuous.html)

Last modified on 2024-07-28