geomhline and geomvline

These geoms add reference lines to a plot: geom_hline() adds horizontal lines and geom_vline() adds vertical lines. Reference lines are useful for marking thresholds, means, or other important values. Unlike most other geoms, these do not inherit x and y aesthetics from the plot, and they do not affect the x and y scales.

Arguments

• plot::GGPlot (optional): a plot object to add this geom to. This is typically used to facilitate creating your ggplot as part of a @chain.
• data (DataFrame): Data to use for this geom. If not provided, the geom will inherit the data from ggplot.
• aes(...): the names of the columns in the DataFrame that will be used in the mapping
• ...: options that are not mapped to a column (passed to Makie.HLines or Makie.VLines)

Specifying Line Positions

Line positions can be specified in two ways:

1. As a fixed value using the yintercept or xintercept argument (for a single line or multiple lines)
2. As an aesthetic mapping to a column in the data (allows different lines in different facets)

Required Aesthetics

For geom_hline():

• yintercept (can be supplied as argument or aesthetic)

For geom_vline():

• xintercept (can be supplied as argument or aesthetic)

Optional Aesthetics (see aes)

• color / colour

Optional Arguments

• yintercept (geom_hline): y-value(s) for horizontal line(s)
• xintercept (geom_vline): x-value(s) for vertical line(s)
• color / colour: line color
• linewidth: line thickness
• linestyle / linetype: line pattern (e.g., :dash, :dot, :dashdot)
• colormap / palette
• alpha

using TidierPlots
using TidierData
using DataFrames

penguins = penguins_data();

Horizontal Lines

Add a single horizontal line at a specific y-value:

ggplot() +
    geom_hline(yintercept = 3)

Add a horizontal line to an existing plot:

ggplot(penguins) +
    geom_point(@aes(x = bill_length_mm, y = bill_depth_mm)) +
    geom_hline(yintercept = 17, color = :red, linewidth = 2)

Add the mean as a reference line:

mean_depth = sum(penguins.bill_depth_mm)/nrow(penguins)

ggplot(penguins) +
    geom_point(@aes(x = bill_length_mm, y = bill_depth_mm)) +
    geom_hline(yintercept = mean_depth, color = :blue, linewidth = 2, linestyle = :dash)

Add multiple horizontal lines:

ggplot() +
    geom_hline(yintercept = [-1, 0, 1, 4], linewidth = 1)

Map yintercept to a column in the data:

df = DataFrame(y = [1, 2, 3, 4])
ggplot(df, @aes(yintercept = y)) +
    geom_hline(color = :steelblue, linewidth = 2)

Vertical Lines

Add a single vertical line:

ggplot() +
    geom_vline(xintercept = 3)

Add a vertical line to highlight a specific x-value:

ggplot(penguins) +
    geom_point(@aes(x = bill_length_mm, y = bill_depth_mm)) +
    geom_vline(xintercept = 45, color = :red, linewidth = 2, linestyle = :dash)

Add multiple vertical lines:

ggplot() +
    geom_vline(xintercept = [-1, 0, 4], color = :purple, linewidth = 1)

Map xintercept to a column:

df = DataFrame(x = [1, 2, 3, 4])
ggplot(df, @aes(xintercept = x)) +
    geom_vline(color = :orange, linewidth = 2)

Combining Both

Use both horizontal and vertical lines together to create reference grids or mark important points:

ggplot(penguins) +
    geom_point(@aes(x = bill_length_mm, y = bill_depth_mm), alpha = 0.5) +
    geom_hline(yintercept = sum(penguins.bill_depth_mm)/nrow(penguins), color = :blue, linestyle = :dash) +
    geom_vline(xintercept = sum(penguins.bill_length_mm)/nrow(penguins), color = :red, linestyle = :dash)

Different line styles:

ggplot(penguins) +
    geom_point(@aes(x = bill_length_mm, y = bill_depth_mm), alpha = 0.3) +
    geom_hline(yintercept = 15, linestyle = :solid, color = :black) +
    geom_hline(yintercept = 17, linestyle = :dash, color = :red) +
    geom_hline(yintercept = 19, linestyle = :dot, color = :blue) +
    geom_hline(yintercept = 21, linestyle = :dashdot, color = :green)

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