Geom line

Line charts are commonly used to display changes of a continuous variable over another continuous variable. This can be used to great effect to see numerical changes over time.

In this page we will create line charts with ggplot2::geom_line(). Through examples we will demonstrate creating:

A line charts with one line.
A line chart with multiple lines each representing a different group.
A stepped line chart.

Tidyverse reference page

Dataset

For demonstration we’ll load the fisheries_long_tbl data from the mgrtibbles package (hyperlink includes install instructions). We will filter to keep only rows from EU, Japan, Peru, and USA, and the year 2000 onwards.

#Load package
library("mgrtibbles")
#mammal_sleep_tbl tibble for demonstration
fisheries_long_tbl<- mgrtibbles::fisheries_long_tbl |>
    #Filter to only retain rows from certain countries
    # and to keep years from 2000 onwards
    dplyr::filter(Entity %in% c("European Union",  "Japan", "Peru", "United States") , 
    Year >= 2000)
fisheries_long_tbl |> dplyr::glimpse()

Rows: 76
Columns: 3
$ Entity      <chr> "European Union", "European Union", "European Union", "Eur…
$ Year        <dbl> 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009…
$ Metric_tons <dbl> 6097232, 6205506, 5865091, 5393438, 5402391, 5193205, 5186…

Single line chart

When creating a line chart only one y value can be assigned to each x value per line. To create a line chart with only one line we will plot the information for Japan.

Create a line chart of Metric_tons (y) against Year (x) for Japan. Additionally, we’ll set linewidth=1 in ggplot::geom_line() to make the line thicker for clarity.

fisheries_long_tbl |>
    #Filter to only retain Japan
    dplyr::filter(Entity=="Japan") |>
    ggplot2::ggplot(aes(x = Year, y = Metric_tons)) +
        ggplot2::geom_line(linewidth = 1)

Although you can create a line chart with multiple x values per y values it becomes impossible to read fully.

The below example plots a line chart with the _tibble without filtering out one Entity or colour grouping by Entity (see below). As there are four entities (EU, Japan, Peru, USA) this means there are four y values per year which are all connected before moving ont the next year.

fisheries_long_tbl |>
    ggplot2::ggplot(aes(x = Year, y = Metric_tons)) +
        ggplot2::geom_step(linewidth = 1)

Multiple lines chart

Line charts are great for showing changes across a time range of several groups. Each group can be represented as a coloured line on the same chart.

Create a Metric_tons (y) against Year (x) line chart for our tibble. In aes() set colour=Entity so each Entity will be represented a by separate coloured line.

fisheries_long_tbl |>
    ggplot2::ggplot(aes(x = Year, y = Metric_tons, colour = Entity)) +
        ggplot2::geom_line(linewidth = 1)

Steps

An optional method for a line chart is a stepped line chart. This can make it a bit easier to compare the individual values between coloured groups.

Create a Metric_tons (y) against Year (x) stepped line chart for our tibble. In aes() set colour=Entity so each Entity will be represented a by separate coloured line.

fisheries_long_tbl |>
    ggplot2::ggplot(aes(x = Year, y = Metric_tons, colour = Entity)) +
        ggplot2::geom_step(linewidth = 1)