# Adding and Labeling a Reference Line

With a lot of graph types, you may want to add a reference line so that the data can be compared to it. For example, perhaps you have a graph that shows growth over time, and want to have a reference line for “no growth” so you can easily see how far things have come. Or perhaps an event happens at a particular time and you want to mark when the event is. Or maybe you just want it to be easy to compare different categories to a mean.

## Keep in Mind

- Make sure that it’s clear what your reference line is. A reader might not guess that it represents a mean, or a particular event, or something else. In some cases, the line extending to a particular value on the x- or y-axis does the job. Other times you might want a direct label.

## Also Consider

- This page will show how to place a line but not how to style it. You may want your line to be dashed, or bold, or a different color. In most cases the stylistic controls for your reference line will be the exact same as those for a regular line graph. See styling line graphs

# Implementations

These implementations will add a line indicating the mean area to the bar graphs found in line graph. They will also show how to place a vertical line, this time at a particular value between the bars, showing how reference lines can be placed on discrete axes as well.

## Python

```
# Load packages
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import seaborn as sns
df = pd.read_csv("https://vincentarelbundock.github.io/Rdatasets/csv/DAAG/Manitoba.lakes.csv", index_col=0)
# Calculate the value where we want the reference line to be, the mean
# Note we could pick any other value here that we wanted
mean_area = np.mean(df['area'])
# This uses pandas' built-in bar plot function, but this uses
# matplotlib under the hood; any other matplotlib bar graph works the same
plt.style.use('seaborn')
ax = df.plot.bar(y='area', legend=False, ylabel='Area', rot=15)
ax.set_title('Area of lakes in Manitoba', loc='left')
# Place the line
plt.axhline(mean_area)
# Look at our result and figure out the appropriate x/y location
# We can figure out visually that it should be a bit above 5000, where
# the line is. But how about x? We can set x by trial and error, or
# note that there are 9 bars and get the x-coordinate of the last one
# using ax.patches.get_x(), and adjust from there
plt.annotate('Mean Area', xy = (ax.patches[8].get_x() - .5, 5500))
# Similarly if we want to position a vertical line, we can use plt.axvline.
# How to position it on a discrete non-numeric x axis?
# place it after the third bar using get_x to find the third bar
# and get_width to move over to the right side of the bar, then a bit more to adjust
plt.axvline(ax.patches[2].get_x() + ax.patches[2].get_width() + .25)
```

## R

In this example, we will place the line’s label using the **ggplot2** function `annotate()`

, which will require us to figure out the annotation’s coordinates ourselves. However, if you prefer, you can use the point-and-click annotation tool ggannotate.

```
library(tidyverse)
df = read_csv("https://vincentarelbundock.github.io/Rdatasets/csv/DAAG/Manitoba.lakes.csv") %>%
rename(Location = ...1)
# Get the reference value we want to mark
mean_area = df %>% pull(area) %>% mean()
# Make the bar plot, and order the bars by height, why not
p <- ggplot(df, aes(x = reorder(Location, -area), y = area)) +
geom_col() +
labs(x = 'Location', y = 'Area') +
# add a horizontal line using geom_hline, specifying its y intercept
geom_hline(yintercept = mean_area)
# Look at our result so far and figure out the appropriate x/y location
# for our annotation.
p
# We can figure out visually that it should be a bit above 5000, where
# the line is. But how about x? We want it over the 9th bar, so we start with 9
# and can adjust from there by changing x or the horizontal justification (hjust)
p <- p + annotate(geom = 'text', label = 'Mean Area', x = 9, y = 5500) +
# Now we can add a vertical reference line with geom_vline
# If we want it between bars 3 and 4, that puts the line at x = 3.5
geom_vline(xintercept = 3.5)
p
```

## Stata

```
import delimited "https://vincentarelbundock.github.io/Rdatasets/csv/DAAG/Manitoba.lakes.csv", clear
rename v1 Location
* Get the reference value we want to mark
summ area
local mean_area = r(mean)
* Make the bar plot, and order the bars by height, why not
* And add a horizontal reference line with yline
graph bar area, over(Location, sort(1)) yti(Area) ///
yline(`mean_area')
* From here, you need to use the Graph Editor
* Right-click the graph, do "Start Graph Editor", and add the annotation
* And you can also add a line object to place a vertical reference line
* If we want to do things ourselves, we need to switch to twoway
* (which we should do here anyway for a method that works with non-bar graphs)
* Unfortunately, twoway bar doesn't like categorical x axes so we need to do some work there
* Put the bars in order and create a labeled numeric variable
* that's in the order we want the bars
gsort -area
g Location_n = _n
* Make sure labutil is installed with ssc install labutil
labmask Location_n, values(Location)
* Get the reference value we want to mark
summ area
local mean_area = r(mean)
* NOW A DILEMMA:
* we can easily add vertical and horizontal lines in twoway with yline and xline (or tline for time series graphs)
* BUT these go BEHIND the graph, not in the foreground
* that's okay for our vertical line between bars 3 and 4 (at 3.5) so let's do that
* but for our horizontal line we'll need to draw it ourselves with function
* which is annoying since we'll have to specify its range by hand
* Start with our basic graph that mimics the graph bar we started with
twoway (bar area Location_n, xti("Location") yti("Area") xlab(1/9, valuelabel) legend(off) ///
xline(3.5)) /// Our vertical line goes between bars 3 and 4, i.e. at 3.5
(function y = `mean_area', range(.5 9.5) /// Now our horizontal line at the mean
)
* Now we can look at our result and see where we think the annotation
* should go. For our x-axis value we have 9 bars so should aim somewhere around 9
twoway (bar area Location_n, xti("Location") yti("Area") xlab(1/9, valuelabel) legend(off) ///
xline(3.5) text(5500 9 "Mean Area")) /// Our vertical line goes between bars 3 and 4, i.e. at 3.5
(function y = `mean_area', range(.5 9.5))
```

## Tableau

To add a reference line in Tableau is easy when it’s a continuous variable. Start by making your graph.

Then, go to the analytics pane and drag a reference line to your graph.

Select the calculation (or value) you want

And the result will be automatically labeled.

For a reference line on a discrete variable, the process is much more involved. See this guide.