Description
For a monotonic transformation that reverses the tonicity of the numbers (low-high -> high-low), secondary axes aren't rendering correctly.
You can see this with a reciprocal transform. The left-most break on top should be close to 1/min(mtcars$wt) or ~0.65. The right-most break should be close to 1/max(mtcars$wt) or ~0.20. For the right-hand transformed axis, the bottom-most break should be close to 1/0.20 or 5 (not 2), and the top-most break should be close to 1/0.65 or ~1.5 (not 5).
Note that these swaps have reversed the intended direction, but still aren't a 1:1 swap of positions.
This is even more obvious with a simple negative transform. Here on the y-axes, -5 maps to (closely, but not exactly) +2, and -2 maps to (closely, but not exactly) +5. And on the x-axes, +2 maps to ~-5. and +5 maps to ~-2.
This issue was noticed in a temperature transformation on a StackOverflow question:
https://stackoverflow.com/questions/54610847/ggplot2-secondary-axis-not-mapping-to-transformation
In both these figures, the values on the left-top axes and bottom-right axes should mirror each other around the diagonal.
library(ggplot2)
#> Warning: package 'ggplot2' was built under R version 3.5.2
ggplot(mtcars, aes(wt, 1/wt)) +
geom_point() +
scale_x_continuous(
sec.axis = sec_axis(trans = ~1/., breaks = seq(0, 1, by = 0.05))
) +
scale_y_continuous(
breaks = seq(0, 1, by = 0.05),
sec.axis = sec_axis(trans = ~1/.)
)
ggplot(mtcars, aes(wt, -wt)) +
geom_point() +
scale_x_continuous(
sec.axis = sec_axis(trans = ~-.)
) +
scale_y_continuous(
sec.axis = sec_axis(trans = ~-.)
)
Session info
sessionInfo()
#> R version 3.5.0 (2018-04-23)
#> Platform: x86_64-w64-mingw32/x64 (64-bit)
#> Running under: Windows 10 x64 (build 16299)
#>
#> Matrix products: default
#>
#> locale:
#> [1] LC_COLLATE=English_United States.1252
#> [2] LC_CTYPE=English_United States.1252
#> [3] LC_MONETARY=English_United States.1252
#> [4] LC_NUMERIC=C
#> [5] LC_TIME=English_United States.1252
#>
#> attached base packages:
#> [1] stats graphics grDevices utils datasets methods base
#>
#> other attached packages:
#> [1] ggplot2_3.1.0
#>
#> loaded via a namespace (and not attached):
#> [1] Rcpp_0.12.17 xml2_1.2.0 bindr_0.1.1 knitr_1.20
#> [5] magrittr_1.5 tidyselect_0.2.4 munsell_0.4.3 colorspace_1.3-2
#> [9] R6_2.2.2 rlang_0.3.1 httr_1.3.1 stringr_1.3.1
#> [13] plyr_1.8.4 dplyr_0.7.5 tools_3.5.0 grid_3.5.0
#> [17] gtable_0.2.0 withr_2.1.2 htmltools_0.3.6 yaml_2.1.19
#> [21] lazyeval_0.2.1 rprojroot_1.3-2 digest_0.6.15 assertthat_0.2.0
#> [25] tibble_1.4.2 bindrcpp_0.2.2 purrr_0.2.4 curl_3.2
#> [29] mime_0.5 glue_1.2.0 evaluate_0.10.1 rmarkdown_1.9
#> [33] labeling_0.3 stringi_1.1.7 compiler_3.5.0 pillar_1.2.3
#> [37] scales_0.5.0 backports_1.1.2 pkgconfig_2.0.1