Line plot for two-way designs using ggplot2

July 24, 2016
By

(This article was first published on blogR, and kindly contributed to R-bloggers)

Want to use R to plot the means and compare differences between groups, but don’t know where to start? This post is for you.

As usual, let’s start with a finished example:

library(dplyr)
library(ggplot2)

pd <- position_dodge(width = 0.2)
mtcars %>%
  mutate(cyl = factor(cyl), am = factor(am, labels = c("automatic", "manual"))) %>% 
  group_by(cyl, am) %>% 
  summarise(hp_mean = mean(hp),
            hp_ci   = 1.96 * sd(hp)/sqrt(n())) %>% 
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_line(aes(linetype = am), position = pd) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1, position = pd, linetype = 1) +
    geom_point(size = 4, position = pd) +
    geom_point(size = 3, position = pd, color = "white") +
    guides(linetype = guide_legend("Transmission")) +
    labs(title = paste("Mean horsepower depending on",
                       "number of cylinders and transmission type.",
                       "Error bars represent 95% Confidence Intervals",
                       sep = "\n"),
         x = "Number of cylinders",
         y = "Gross horsepower") +
  theme(
    panel.background = element_rect(fill = "white"),
    legend.key  = element_rect(fill = "white"),
    axis.line.x = element_line(colour = "black", size = 1),
    axis.line.y = element_line(colour = "black", size = 1)
  )

posts-init-example-1.png

Let’s break this down.

 Summarising the data

The first challenge is the data. When attempting to make a plot like this in R, I’ve noticed that many people (myself included) start by searching for how to make line plots, etc. in R. This is natural. However, for those who are relatively new to R and are more comfortable with the likes of SPSS, being able to produce the plot isn’t necessarily the place to start. Rather, the first thing you should think about is transforming your data into the points that are going to be plotted.

Imagine the plot you’re about to produce. In our example, each point represents the mean horsepower of some group (based on the number of cylinders and transmission), and error bars represent the 95% confidence intervals. We’re not plotting every point in our data set; we’re plotting very specific summary statistics. So, although programmes like SPSS do this summary behind the scenes for us (and there are ways to make this happen automatically in R), I find that it’s best to explicitly calculate these values as a sanity check and to better understand our data. Let’s get to it.

We’ll be working with the mtcars dataset which comes with R and contains various information about 32 cars from 1974. Run ?mtcars from your console to learn more. Here, we’re interested in plotting the mean gross horsepower (hp) as a function of two categorical variables: the number of cylinders (cyl) and the transmission type (am). As a quick aside, note that this example will, therefore, be directly applicable to plotting the mean of a continuous variable grouped by two categorical variables (e.g., from a two-way experimental design).

Let’s focus on the relevant columns and convert the grouping variables to factors. To do this, we’ll use select() and mutate() from the dplyr package. We won’t bother giving cyl labels (as they’re already the numbers we want), but we’ll label am so we don’t need to worry about converting 0s and 1s in our head.

library(dplyr)
d <- mtcars %>%
       select(cyl, am, hp) %>%  # select relevant variables
       mutate(cyl = factor(cyl),  # Convert grouping variables to factors
              am  = factor(am, labels = c("automatic", "manual")))

head(d)
#>   cyl        am  hp
#> 1   6    manual 110
#> 2   6    manual 110
#> 3   4    manual  93
#> 4   6 automatic 110
#> 5   8 automatic 175
#> 6   6 automatic 105

We need the mean horsepower for each cylinder-transmission combination. This is best achieved with a couple more functions from dplyr: group the data by the factors with group_by(), and summarise() to compute the means:

d %>% 
  group_by(cyl, am) %>%
  summarise(hp_mean = mean(hp))
#> Source: local data frame [6 x 3]
#> Groups: cyl [?]
#> 
#>      cyl        am   hp_mean
#>            
#> 1      4 automatic  84.66667
#> 2      4    manual  81.87500
#> 3      6 automatic 115.25000
#> 4      6    manual 131.66667
#> 5      8 automatic 194.16667
#> 6      8    manual 299.50000

We now have a mean column which represents where the points will go on in the plot. Let’s also compute the distance from the mean to the 95% confidence interval (hp_ci) in anticipation of the error bars and save the resulting data frame:

sum_d <- d %>% 
          group_by(cyl, am) %>%
          summarise(hp_mean = mean(hp),
                    hp_ci   = 1.96 * sd(hp)/sqrt(n()))
sum_d
#> Source: local data frame [6 x 4]
#> Groups: cyl [?]
#> 
#>      cyl        am   hp_mean     hp_ci
#>                 
#> 1      4 automatic  84.66667 22.242138
#> 2      4    manual  81.87500 15.699402
#> 3      6 automatic 115.25000  8.995204
#> 4      6    manual 131.66667 42.466667
#> 5      8 automatic 194.16667 18.875105
#> 6      8    manual 299.50000 69.580000

For those a little rusty with statistics, sd(hp)/sqrt(n()) gives us the standard error of the mean, and multiplying this by 1.96 gives us the distance from the mean to the 95% interval boundary (which we then add or subtract from the mean).

Great, we’ve now got a data frame of the information we need to produce the plot.

 Creating the plot

To create the plot, we’ll use the ggplot2 package. When you create a plot with ggplot2, you build up layers of graphics. It’s important to keep this idea of layering in mind as we gradually build the plot.

 Setup

To start, we’ll set up a blank plot canvas with relevant x and y-axes using ggplot():

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean))

posts-plot-canvas-1.png

 Adding points

Next, we want to represent our data in our plot. ggplot2 uses various geoms to do this, which are layered into the plot using +. Let’s start with the points themselves by using geom_point():

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean)) +
    geom_point()

posts-plot-points-1.png

 Adding lines

Next we’ll add the lines with geom_line():

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean)) +
    geom_point() +
    geom_line()

posts-plot-lines-1.png

Hmm, that doesn’t seem right! What’s going on here? The problem is that we haven’t specified that the lines should be grouped by transmission, so it’s just using the already provided number of cylinders. To handle this, we assign the group and linetype aesthetics to our second categorical variable, am. Note that group is handled in ggplot, but linetype is in geom_line(). These can be moved around, but having group in ggplot is important for the position adjustment discussed later.

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_point() +
    geom_line(aes(linetype = am))

posts-plot-lines2-1.png

 Adding error bars

The last visual element to add is the error bars using geom_errorbar(), which requires us to specify ymin and ymax for each error bar. I like to do the calculation within this function as follows:

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_point() +
    geom_line(aes(linetype = am)) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci))

posts-plot-error-1.png

OK, not looking perfect yet, but let’s quickly discuss how the error bars are working. By declaring ymin = hp_mean - hp_ci, we’re saying that the minimum level of the error bar for each point will be hp_mean (which is the position of the point itself) minus the distance to the confidence interval boundary (one standard error multiplied by 1.96). For ymax, we add rather than subtract, thus giving us the upper bound for the 95% confidence interval. A quick note, you can change these error bar values to whatever suits you (e.g., could drop * 1.96 from the original calculation for a single standard error).

 Make it pretty

We’ve got all the visual elements we need. Now, it’s all about making the plot look nice.

 Error bars

Let’s start by making those error bars a little less intense by reducing their width:

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_point() +
    geom_line(aes(linetype = am)) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1)  # Reduce the width of the error bars

posts-plot-pretty-error-1.png

 Points

Next, let’s get the points to be big white circles with black outlines. We’ll start by adjusting the size and color of the points:

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_point(size = 3, color = "white") +  # Increase point size and change to white.
    geom_line(aes(linetype = am)) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1)

posts-plot-pretty-points-1.png

What’s gone wrong here? Well, a couple of things. Firstly, the points are being obscured by the lines and error bars. Secondly, the points don’t have a black outline. Handling the first problem is easier. Recall that we’re building the plot up with layers. So ggplot2 places each layer on top of previous ones. Thus, the order in which we create the geoms will determine which layers fall on top of the others. We created the geom_point() layer before geom_line() and geom_errorbar(), so the points appear behind them. Therefore, the fix is easy – add the points AFTER the other layers as such:

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_line(aes(linetype = am)) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1) +
    geom_point(size = 3, color = "white")

posts-plot-pretty-points2-1.png

Now, what about those black outlines? Thanks to the layering, we can put black points, which are slightly larger in size than the white points, in the plot before, and thus behind, the white points. Doing this creates the effect that the points have an outline. Admittedly, this is a little bit of trickery for which a better approach might exist, but this is how I do it:

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_line(aes(linetype = am)) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1) +
    geom_point(size = 4) +  # Add black points first; larger size than white points
    geom_point(size = 3, color = "white")

posts-plot-pretty-points3-1.png

If you were to comment out the last line of the above code, you’ll see that it’s just large black points that are being plotted, and over which we’re putting smaller white ones.

 Position

Next, we’ll adjust the position of the points so that they, and their error bars, don’t overlap. First, remember that there are multiple geom layers: the lines, the error bars, and two point layers. Each of these has to be adjusted separately. We can do this inside our pipe, but to save the effort when tweaking the value, let’s set a single adjustment variable as follows:

pd <- position_dodge(width = 0.2)

Now, we add position = pd into all the geom layers:

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_line(aes(linetype = am), position = pd) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1, position = pd) +
    geom_point(size = 4, position = pd) +
    geom_point(size = 3, color = "white", position = pd)

posts-plot-pretty-position-1.png

 Labels

Almost there. Now, we need to fix the labels of the axes and the legend and add a title. The axes and title can be handled together using labs(). The legend, however, needs to be managed separately using guides(). To create a multi-line title, I use paste(), and separate each section (sep =) of text with "\n", which represents a new line.

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_line(aes(linetype = am), position = pd) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1, position = pd) +
    geom_point(size = 4, position = pd) +
    geom_point(size = 3, color = "white", position = pd) + 
    guides(linetype = guide_legend("Transmission")) +   # Change the legend title
    labs(title = paste("Mean horsepower depending on",  # Add a multi-line title
                       "number of cylinders and transmission type.",
                       "Error bars represent 95% Confidence Intervals",
                       sep = "\n"),
         x = "Number of cylinders",  # Change x-axis label
         y = "Gross horsepower")     # Change y-axis label

posts-plot-pretty-labs-1.png

 Theme

The final piece of the puzzle is to add a theme that removes the background grid and adds some solid lines to the x and y-axes:

sum_d %>%
  ggplot(aes(x = cyl, y = hp_mean, group = am)) +
    geom_line(aes(linetype = am), position = pd) +
    geom_errorbar(aes(ymin = hp_mean - hp_ci, ymax = hp_mean + hp_ci),
                  width = .1, position = pd) +
    geom_point(size = 4, position = pd) +
    geom_point(size = 3, color = "white", position = pd) + 
    guides(linetype = guide_legend("Transmission")) +
    labs(title = paste("Mean horsepower depending on",
                       "number of cylinders and transmission type",
                       "Error bars represent 95% Confidence Intervals",
                       sep = "\n"),
         x = "Number of cylinders",
         y = "Gross horsepower") +
  theme(
    panel.background = element_rect(fill = "white"),         # Set plot background to white
    legend.key  = element_rect(fill = "white"),              # Set legend item backgrounds to white
    axis.line.x = element_line(colour = "black", size = 1),  # Add line to x axis
    axis.line.y = element_line(colour = "black", size = 1)   # Add line to y axis
  )

posts-plot-pretty-theme-1.png

The only difference between this and the example at the beginning is that the data preparation (computing mean and confidence interval distance) is handled within a single pipe.

 Sign off

Thanks for reading and I hope this was useful for you.

For updates of recent blog posts, follow @drsimonj on Twitter, or email me at [email protected] to get in touch.

If you’d like the code that produced this blog, check out my GitHub repository, blogR.

To leave a comment for the author, please follow the link and comment on their blog: blogR.

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