Default aesthetics for stats can create unexpected legend titles

[datalorax]

I've run into some unexpected behavior when using geom_density_2d.

If I run

library(ggplot2)
ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  geom_smooth(aes(color = drv)) 

The legend title corresponds to the variable name (drv).

But if I add a geom_density_2d layer that I want plotted under the smooths, the legend title changes to "colour".

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  geom_density2d() +
  geom_smooth(aes(color = drv)) 

I'm aware that I could change this manually with guides(color = guide_legend(title = "drv")) and so it's not that big of a deal, but it did seem odd to me.

[clauswilke]

As far as I can tell, this is a general problem whenever a stat defines a default aesthetic that the user hasn't mapped but does so in a later layer. This is not specific to geom_density2d().

Alternative reprex, with geom_density(), which calls stat_density(), which creates a fill aesthetic:

library(ggplot2)
ggplot(mpg, aes(displ)) +
  geom_density() +
  geom_point(aes(y = .1, fill = drv), shape = 21)

[clauswilke]

See also:

library(ggplot2)
p <- ggplot(mpg, aes(displ)) +
  geom_density() +
  geom_point(aes(y = .1, fill = drv), shape = 21)
ggplot_build(p)$plot$labels
#> $x
#> [1] "displ"
#> 
#> $y
#> [1] "density"
#> 
#> $fill
#> [1] "fill"

p <- ggplot(mpg, aes(displ)) +
  geom_density()# +
  #geom_point(aes(y = .1, fill = drv), shape = 21)
ggplot_build(p)$plot$labels
#> $x
#> [1] "displ"
#> 
#> $y
#> [1] "density"
#> 
#> $fill
#> [1] "fill"

p <- ggplot(mpg, aes(displ)) +
  #geom_density() +
  geom_point(aes(y = .1, fill = drv), shape = 21)
ggplot_build(p)$plot$labels
#> $x
#> [1] "displ"
#> 
#> $y
#> [1] "y"
#> 
#> $fill
#> [1] "drv"

Note that geom_density() also overwrites the y axis label, and we probably want that to be the case even when we add points with a y aesthetic, so it's not immediately clear what the correct behavior is.

[hadley]

Is this a long standing issue, or did it get introduced in 2.3?

[clauswilke]

Something seems to have changed. Output with ggplot2 2.2.1:

library(ggplot2)
p <- ggplot(mpg, aes(displ)) +
  geom_density() +
  geom_point(aes(y = .1, fill = drv), shape = 21)
ggplot_build(p)$plot$labels
#> $x
#> [1] "displ"
#> 
#> $y
#> [1] "density"
#> 
#> $fill
#> [1] "NA"

Created on 2018-05-28 by the reprex package (v0.2.0).

Session info

devtools::session_info()
#> Session info -------------------------------------------------------------
#>  setting  value                       
#>  version  R version 3.5.0 (2018-04-23)
#>  system   x86_64, darwin15.6.0        
#>  ui       X11                         
#>  language (EN)                        
#>  collate  en_US.UTF-8                 
#>  tz       America/Chicago             
#>  date     2018-05-28
#> Packages -----------------------------------------------------------------
#>  package    * version    date       source                          
#>  backports    1.1.2      2017-12-13 CRAN (R 3.5.0)                  
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#>  ggplot2    * 2.2.1      2016-12-30 CRAN (R 3.5.0)                  
#>  graphics   * 3.5.0      2018-04-24 local                           
#>  grDevices  * 3.5.0      2018-04-24 local                           
#>  grid         3.5.0      2018-04-24 local                           
#>  gtable       0.2.0      2016-02-26 CRAN (R 3.5.0)                  
#>  htmltools    0.3.6      2017-04-28 CRAN (R 3.5.0)                  
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#>  labeling     0.3        2014-08-23 CRAN (R 3.5.0)                  
#>  lazyeval     0.2.1      2017-10-29 CRAN (R 3.5.0)                  
#>  magrittr     1.5        2014-11-22 CRAN (R 3.5.0)                  
#>  memoise      1.1.0      2017-04-21 CRAN (R 3.5.0)                  
#>  methods    * 3.5.0      2018-04-24 local                           
#>  munsell      0.4.3      2016-02-13 CRAN (R 3.5.0)                  
#>  pillar       1.2.1      2018-02-27 CRAN (R 3.5.0)                  
#>  plyr         1.8.4      2016-06-08 CRAN (R 3.5.0)                  
#>  Rcpp         0.12.16    2018-03-13 CRAN (R 3.5.0)                  
#>  rlang        0.2.0.9001 2018-05-15 Github (r-lib/rlang@854174a)    
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#>  scales       0.5.0.9000 2018-05-15 Github (hadley/scales@d767915)  
#>  stats      * 3.5.0      2018-04-24 local                           
#>  stringi      1.1.7      2018-03-12 CRAN (R 3.5.0)                  
#>  stringr      1.3.0      2018-02-19 CRAN (R 3.5.0)                  
#>  tibble       1.4.2      2018-01-22 CRAN (R 3.5.0)                  
#>  tools        3.5.0      2018-04-24 local                           
#>  utils      * 3.5.0      2018-04-24 local                           
#>  withr        2.1.2      2018-05-15 Github (jimhester/withr@79d7b0d)
#>  yaml         2.1.18     2018-03-08 CRAN (R 3.5.0)

To highlight the difference, in ggplot2 2.2.1:

#> $fill
#> [1] "NA"

instead of ggplot2 2.3.0:

#> $fill
#> [1] "fill"

[clauswilke]

However, the resulting 2.2.1 behavior wasn't necessarily better:

library(ggplot2)
ggplot(mpg, aes(displ)) +
  geom_density() +
  geom_point(aes(y = .1, fill = drv), shape = 21)

Created on 2018-05-28 by the reprex package (v0.2.0).

Session info

devtools::session_info()
#> Session info -------------------------------------------------------------
#>  setting  value                       
#>  version  R version 3.5.0 (2018-04-23)
#>  system   x86_64, darwin15.6.0        
#>  ui       X11                         
#>  language (EN)                        
#>  collate  en_US.UTF-8                 
#>  tz       America/Chicago             
#>  date     2018-05-28
#> Packages -----------------------------------------------------------------
#>  package    * version    date       source                          
#>  backports    1.1.2      2017-12-13 CRAN (R 3.5.0)                  
#>  base       * 3.5.0      2018-04-24 local                           
#>  colorspace   1.4-0      2017-12-23 R-Forge (R 3.5.0)               
#>  compiler     3.5.0      2018-04-24 local                           
#>  curl         3.1        2017-12-12 CRAN (R 3.5.0)                  
#>  datasets   * 3.5.0      2018-04-24 local                           
#>  devtools     1.13.5     2018-02-18 CRAN (R 3.5.0)                  
#>  digest       0.6.15     2018-01-28 CRAN (R 3.5.0)                  
#>  evaluate     0.10.1     2017-06-24 CRAN (R 3.5.0)                  
#>  ggplot2    * 2.2.1      2016-12-30 CRAN (R 3.5.0)                  
#>  graphics   * 3.5.0      2018-04-24 local                           
#>  grDevices  * 3.5.0      2018-04-24 local                           
#>  grid         3.5.0      2018-04-24 local                           
#>  gtable       0.2.0      2016-02-26 CRAN (R 3.5.0)                  
#>  htmltools    0.3.6      2017-04-28 CRAN (R 3.5.0)                  
#>  httr         1.3.1      2017-08-20 CRAN (R 3.5.0)                  
#>  knitr        1.20       2018-02-20 CRAN (R 3.5.0)                  
#>  labeling     0.3        2014-08-23 CRAN (R 3.5.0)                  
#>  lazyeval     0.2.1      2017-10-29 CRAN (R 3.5.0)                  
#>  magrittr     1.5        2014-11-22 CRAN (R 3.5.0)                  
#>  memoise      1.1.0      2017-04-21 CRAN (R 3.5.0)                  
#>  methods    * 3.5.0      2018-04-24 local                           
#>  mime         0.5        2016-07-07 CRAN (R 3.5.0)                  
#>  munsell      0.4.3      2016-02-13 CRAN (R 3.5.0)                  
#>  pillar       1.2.1      2018-02-27 CRAN (R 3.5.0)                  
#>  plyr         1.8.4      2016-06-08 CRAN (R 3.5.0)                  
#>  R6           2.2.2      2017-06-17 CRAN (R 3.5.0)                  
#>  Rcpp         0.12.16    2018-03-13 CRAN (R 3.5.0)                  
#>  rlang        0.2.0.9001 2018-05-15 Github (r-lib/rlang@854174a)    
#>  rmarkdown    1.9        2018-03-01 CRAN (R 3.5.0)                  
#>  rprojroot    1.3-2      2018-01-03 CRAN (R 3.5.0)                  
#>  scales       0.5.0.9000 2018-05-15 Github (hadley/scales@d767915)  
#>  stats      * 3.5.0      2018-04-24 local                           
#>  stringi      1.1.7      2018-03-12 CRAN (R 3.5.0)                  
#>  stringr      1.3.0      2018-02-19 CRAN (R 3.5.0)                  
#>  tibble       1.4.2      2018-01-22 CRAN (R 3.5.0)                  
#>  tools        3.5.0      2018-04-24 local                           
#>  utils      * 3.5.0      2018-04-24 local                           
#>  withr        2.1.2      2018-05-15 Github (jimhester/withr@79d7b0d)
#>  xml2         1.2.0      2018-01-24 CRAN (R 3.5.0)                  
#>  yaml         2.1.18     2018-03-08 CRAN (R 3.5.0)

[hadley]

Ah ok, let's consider for the next release then. I think the change is correct, but we need better behaviour when combining labels across multiple layers.

[clauswilke]

For future reference, this is the code that makes labels for default aesthetics of stats:

ggplot2/R/plot-construction.r

Lines 159 to 162 in 86c41ac

	mapping <- make_labels(object$mapping)
	default <- make_labels(object$stat$default_aes)
	new_labels <- defaults(mapping, default)
	plot$labels <- defaults(plot$labels, new_labels)