Make legend glyphs configurable

[clauswilke]

Currently all geoms have a hard-coded legend glyph that can't be changed without some knowledge about the inner workings of ggplot2. It would be helpful if the glyphs could be configurable. Associated with such a change, we might want to provide a broader array of different glyphs. One example is the stylized time series glyph of PR #3124.

It seems to me the easiest way to achieve this modification is to provide an additional parameter draw_key to the constructor of each geom. The downside is that it requires a modification of each individual geom_*() and will not automatically propagate to extension packages. The upside is that the modification required is rather minor. Example implementation follows.

library(ggplot2)

# alternative key glyph
draw_key_timeseries <- function(data, params, size) {
  data$linetype[is.na(data$linetype)] <- 0

  grid::linesGrob(
    x = c(0, 0.4, 0.6, 1),
    y = c(0.1, 0.6, 0.4, 0.9),
    gp = grid::gpar(
      col = scales::alpha(data$colour, data$alpha),
      lwd = data$size * .pt,
      lty = data$linetype,
      lineend = "butt"
    )
  )
}

# helper function
set_draw_key <- function(geom, draw_key = NULL) {
  if (is.null(draw_key)) {
    return(geom)
  }
  if (is.character(draw_key)) {
    draw_key <- paste0("draw_key_", draw_key)
  }
  draw_key <- match.fun(draw_key)
  
  ggproto("", geom, draw_key = draw_key)
}

# version of `geom_line()` with modifiable draw key
geom_line2 <- function(mapping = NULL, data = NULL, stat = "identity",
                       position = "identity", na.rm = FALSE,
                       show.legend = NA, inherit.aes = TRUE, draw_key = NULL,
                       ...) {
  layer(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = set_draw_key(GeomLine, draw_key),
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      na.rm = na.rm,
      ...
    )
  )
}

# usage
base <- ggplot(data.frame(x = 1:3, y = 3:1), aes(x, y))
base + geom_line2(aes(color = "default"))

base + geom_line2(aes(color = "modified"), draw_key = "timeseries")

base + geom_line2(aes(color = "modified"), draw_key = draw_key_timeseries)

^{Created on 2019-02-15 by the reprex package (v0.2.1)}

[clauswilke]

One note: If we go this route, we may want to make the draw_key_*() functions more robust to missing aesthetics.

base + geom_line2(aes(color = "modified"), draw_key = "vpath")

base + geom_line2(aes(color = "modified"), draw_key = "abline")

base + geom_line2(aes(color = "modified"), draw_key = "point")
#> Error in check.length(gparname): 'gpar' element 'fontsize' must not be length 0

base + geom_line2(aes(color = "modified"), draw_key = "polygon")
#> Error in check.length("fill"): 'gpar' element 'fill' must not be length 0

[jemus42]

I often find myself wanting to have the legend glyphs just solid colors, for example in cases where I'm plotting points with different colors and no other geoms – in that case I would prefer the legend to simply indicate the different colors instead of indicating colors and a geometric shape that is not required to be identifiable.

If this proposed change is implemented, would that mean I could choose to override legend keys with "just give me a solid box" easily?

[clauswilke]

yes.

[baptiste]

I've seen plots where the legend reproduced a miniature trace of the data it's referring to (e.g for time-series); it worked quite well.

Kind of related to this, I've come to believe that guides could be based on a trimmed down grammar of graphics. Basically, a guide would become a kind of mini ggplot itself.

https://gist.github.com/baptiste/960186a01ca3c48b112f4a15fe013884

It might be impractical in terms of code re-use, I'm not sure; obviously for speed and other reasons one would want a drastically-simplified framework (no grids, strips, scale training, stats, transformations, etc. — only the already-processed data would be passed, and a subset at that).

The output of ggplot_build contains data that could be relatively easily processed by a simplified plotting routine, but that still contains axes, theming, facetting, and other customisations.

[clauswilke]

@baptiste I think what you're bringing up is a separate issue. This specific issue is meant to make a small modification to the existing guide code. What you're proposing would require a complete overhaul of that code. I'm not opposed to that in principle, but I think it deserves its own issue.

@hadley @thomasp85 @yutannihilation Could you take a quick look at my functions set_draw_key() and geom_line2() at the top of this issue and let me know if you have any concerns about this approach to making legend keys configurable?

[yutannihilation]

Looks OK.

But, I'm wondering if it is really what we want to specify draw_key on individual geoms. For example, do we really need different lines for different line geoms in the plot below?

Details

library(ggplot2)

# alternative key glyph
draw_key_timeseries <- function(data, params, size) {
  data$linetype[is.na(data$linetype)] <- 0
  
  grid::linesGrob(
    x = c(0, 0.4, 0.6, 1),
    y = c(0.1, 0.6, 0.4, 0.9),
    gp = grid::gpar(
      col = scales::alpha(data$colour, data$alpha),
      lwd = data$size * .pt,
      lty = data$linetype,
      lineend = "butt"
    )
  )
}

# helper function
set_draw_key <- function(geom, draw_key = NULL) {
  if (is.null(draw_key)) {
    return(geom)
  }
  if (is.character(draw_key)) {
    draw_key <- paste0("draw_key_", draw_key)
  }
  draw_key <- match.fun(draw_key)
  
  ggproto("", geom, draw_key = draw_key)
}

# version of `geom_line()` with modifiable draw key
geom_line2 <- function(mapping = NULL, data = NULL, stat = "identity",
                       position = "identity", na.rm = FALSE,
                       show.legend = NA, inherit.aes = TRUE, draw_key = NULL,
                       ...) {
  layer(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = set_draw_key(GeomLine, draw_key),
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      na.rm = na.rm,
      ...
    )
  )
}

# usage
base <- ggplot(data.frame(x = 1:3, y = 3:1), aes(x, y))

base +
  geom_line(aes(color = "base")) +
  geom_line2(aes(color = "modified"), draw_key = "timeseries")

^{Created on 2019-03-18 by the reprex package (v0.2.1.9000)}

One idea is to have a list of draw keys on ggplot_global and set_draw_key("line", draw_key_timeseries) will override default draw_key() all line-alike geoms. Probably, something like get_draw_key(layer$geom$legend_type()) would be called here:

ggplot2/R/guide-legend.r

Line 286 in ae1c8f0

draw_key = layer$geom$draw_key,

(I'm posting this just for discussion. I don't have any strong opinions yet)

[yutannihilation]

Sorry, I mistakenly closed the issue...

[clauswilke]

I am not too worried about having to specify a draw key multiple times if somebody uses the same geom multiple times in a plot. That's not any different than, for example, setting the same scale name in all scales (e.g. color, fill, shape) to ensure the legends merge correctly.

We could talk about making draw keys globally configurable, but I feel that's a secondary issue that we can provide in addition to having localized customization, not instead.

[clauswilke]

Actually, changing draw keys globally is trivially possible by modifying the geom. We could add a convenience function that encapsulates this modification if we wanted to.

Details

library(ggplot2)

# alternative key glyph
draw_key_timeseries <- function(data, params, size) {
  data$linetype[is.na(data$linetype)] <- 0
  
  grid::linesGrob(
    x = c(0, 0.4, 0.6, 1),
    y = c(0.1, 0.6, 0.4, 0.9),
    gp = grid::gpar(
      col = scales::alpha(data$colour, data$alpha),
      lwd = data$size * .pt,
      lty = data$linetype,
      lineend = "butt"
    )
  )
}

# change draw key globally
GeomLine$draw_key <- draw_key_timeseries

ggplot(data.frame(x = 1:3, y = 3:1), aes(x, y)) +
  geom_line(aes(color = "base")) +
  geom_line(aes(y = 4-y, color = "modified"))

^{Created on 2019-03-17 by the reprex package (v0.2.1)}

[yutannihilation]

We could talk about making draw keys globally configurable, but I feel that's a secondary issue that we can provide in addition to having localized customization, not instead.

That's convincing, thanks. Then, I have no concern here.

One thing, to be clear, what I was trying to see was not to set the draw_key of individual Geoms, but to override draw_key_*() (e.g. draw_key_line()) globally, so that it might be possible for themes to provide custom legends globally.

[clauswilke]

Actually, now that I think about it, maybe starting with just the function that modifies geoms globally is the way to go. It would be minimally invasive. (I realize I use the same function name set_draw_key() for a different purpose now.)

library(ggplot2)

# function to adjust draw key globally
set_draw_key <- function(geom, draw_key) {
  geom <- ggplot2:::check_subclass(geom, "Geom", env = parent.frame())

  if (is.character(draw_key)) {
    draw_key <- paste0("draw_key_", draw_key)
  }
  draw_key <- match.fun(draw_key)
  
  geom$draw_key <- draw_key
}

# alternative key glyph
draw_key_timeseries <- function(data, params, size) {
  data$linetype[is.na(data$linetype)] <- 0
  
  grid::linesGrob(
    x = c(0, 0.4, 0.6, 1),
    y = c(0.1, 0.6, 0.4, 0.9),
    gp = grid::gpar(
      col = scales::alpha(data$colour, data$alpha),
      lwd = data$size * .pt,
      lty = data$linetype,
      lineend = "butt"
    )
  )
}

set_draw_key("line", "timeseries")
ggplot(data.frame(x = 1:3, y = 3:1), aes(x, y, color = "line")) + geom_line()

set_draw_key("line", "path")
ggplot(data.frame(x = 1:3, y = 3:1), aes(x, y, color = "line")) + geom_line()

^{Created on 2019-03-17 by the reprex package (v0.2.1)}

[yutannihilation]

Can we keep the default draw_key function so that we can reset by set_draw_key("line", NULL)?