# Trace lines

#### June 23 2022

## Basic Usage

`geom_path_trace()`, `geom_line_trace()`, and `geom_step_trace()` are similar to `ggplot2::geom_path()`, `ggplot2::geom_line()`, and `ggplot2::geom_step()`, but also include the ability to highlight data points of interest. This is particularly useful when working with dense datasets that are prone to overplotting. These geoms accept normal ggplot2 graphical parameters with some modifications. `fill` controls the color of the center line, `color` controls the outline color, and `stroke` controls outline width, similar to how filled shapes are modified for other ggplot2 geoms. Additional parameters including `size`, `alpha`, `linetype`, `linejoin`, `lineend` and `linemitre` are also accepted.

    library(ggplot2)
    library(ggtrace)

    p <- ggplot(
      stocks,
      aes(day, value, fill = name)
    ) +
      theme_minimal()

    p +
      geom_line_trace(
        color    = "black",
        size     = 1,
        stroke   = 0.5,
        linetype = 1,
        alpha    = 1
      )

## Aesthetics

Like other ggplot2 geoms, variables can be mapped to aesthetic attributes to modify the outline appearance.

    p <- ggplot(
      stocks,
      aes(day, value, color = name)
    ) +
      theme_minimal()

    p +
      geom_line_trace(stroke = 1)

Aesthetics can be further modified using the ggplot2 `scale_*()` functions.

    clrs <- c(
      CAC  = "#E69F00",
      DAX  = "#0072B2",
      FTSE = "#009E73",
      SMI  = "#56B4E9"
    )

    p +
      geom_line_trace(stroke = 1) +
      scale_color_manual(values = clrs)

## Position

The “position” of the outline can be modified with the `trace_position` parameter. This can be “all” or a predicate selecting the data points to outline. By default all groups are outlined.

If a predicate is passed to `trace_position`, it must evaluate to `TRUE` or `FALSE` within the context of the input data.

    p <- ggplot(
      stocks,
      aes(day, value, color = name)
    ) +
      theme_minimal()

    p +
      geom_line_trace(
        trace_position = day > 1500,
        stroke         = 1
      )

The appearance of background lines can be modified by passing a named list of parameters to the `background_params` argument.

    p +
      geom_line_trace(
        trace_position    = day < 500 | value > 4000,
        stroke            = 1,
        background_params = list(color = NA, fill = "grey75")
      )

Here we only highlight names where the second letter is “A”.

    p +
      geom_line_trace(
        trace_position    = grepl("^[A-Z]A", name),
        stroke            = 1,
        background_params = list(color = NA, linetype = 2)
      )

Outlines can be removed by setting `color` to `NA`.

    p <- ggplot(
      stocks,
      aes(day, value, fill = name)
    ) +
      theme_minimal()

    p +
      geom_line_trace(
        trace_position    = grepl("^[A-Z]A", name),
        color             = NA,
        background_params = list(fill = "grey75")
      )