Perfectly Aligning Axis Titles to Axis Edges

When creating plots with ggplot2, aligning the axis titles to the edges of the plot can be a bit tricky. The functions hjust and vjust are used to define alignment relatively to the entire plot, but what if we want the alignment to be relative to the axis lines themselves?

Understanding Alignment Functions

In ggplot2, the alignment functions hjust and vjust are used to position text elements (such as axis titles) relative to the layout of the plot. The values for these functions can range from 0 (left) to 1 (right), with 0.5 being centered.

When using hjust or vjust, it’s essential to understand that these functions are applied to the entire plot, not just the axis lines. This means that if we want to align an axis title to a specific edge of the plot, we need to use additional functions and techniques to achieve this alignment.

Fixing the Origin

To make it easier to align axis titles to the edges of the plot, we first need to force the plot to start at the origin. This can be done by setting expand_limits to 0, 0, which means that the minimum values for both the x and y axes are set to 0. We also need to define the scale_x_continuous and scale_y_continuous functions with an expand value of c(0, 0), which sets the range of each axis.

expand_limits(x = 0, y = 0)
scale_x_continuous(expand = c(0, 0))
scale_y_continuous(expand = c(0, 0))

Adjusting Axis Titles

Once we’ve fixed the origin, we can adjust the axis titles using the theme() function and its various elements. Specifically, we’re interested in the axis.title.x and axis.title.y elements.

For the x-axis title, we want to left-align it relative to the beginning of the x-axis line. We can achieve this by setting hjust = 0. For the y-axis title, we want to top-align and rotate it relative to the end of the y-axis line. We can do this by setting angle = 90 (which rotates the text 90 degrees) and hjust = 1.

theme(
  axis.title.x = element_text(hjust = 0, margin = margin(t=6)),
  axis.title.y = element_text(angle = 90, hjust = 1, margin=margin(r=6))
)

Adding Axis Titles

After adjusting the axis titles, we can add them to the plot using the xlab() and ylab() functions.

xlab("This is the x axis") + 
ylab("This is the y axis")

Increasing Plot Margin

Finally, if our last label text gets cut off by the margin of the plot, we can increase the plot margin to handle this. We can do this by adding plot.margin = margin(1, 1, 1, 1, "cm") to the theme() function.

plot.margin = margin(1, 1, 1, 1, "cm")

The Full Code

Here’s the complete code with all the adjustments:

library(ggplot2)

df <- data.frame(
  x = c(0, 50, 100),
  y = c(20, 40, 80)
)

graph <- ggplot(data=df, mapping = aes(x=x, y=y)) +
  geom_line() +
  expand_limits(x = 0, y = 0) +
  scale_x_continuous(expand = c(0,0), limits = c(min(df$x), max(df$x))) +
  scale_y_continuous(expand = c(0,0), limits = c(min(df$y), max(df$y))) +
  theme(
    axis.title.x = element_text(hjust = 0, margin = margin(t=6)),
    axis.title.y = element_text(angle = 90, hjust = 1, margin=margin(r=6)),
    plot.margin = margin(1, 1, 1, 1, "cm")
  ) +
  xlab("This is the x axis") + 
  ylab("This is the y axis")

graph

By following these steps and using the right combination of functions and techniques, we can perfectly align our axis titles to the edges of our plot.

Last modified on 2024-07-11