weird behavious when specifying `breaks` in `scale_x_continuous`

[iagogv3]

Are the grid vertical lines around 2 and 6 expected?

library(ggplot2)
p1 <- ggplot(mpg, aes(displ, hwy)) +
    geom_point()
p1 + scale_x_continuous(breaks = c(2, 2.25, 4, 6)) + theme_bw()

sessionInfo()
#> R version 3.6.3 (2020-02-29)
#> Platform: x86_64-pc-linux-gnu (64-bit)
#> Running under: Debian GNU/Linux 9 (stretch)
#> 
#> Matrix products: default
#> BLAS:   /usr/lib/openblas-base/libblas.so.3
#> LAPACK: /usr/lib/libopenblasp-r0.2.19.so
#> 
#> locale:
#>  [1] LC_CTYPE=en_GB.UTF-8       LC_NUMERIC=C              
#>  [3] LC_TIME=en_GB.UTF-8        LC_COLLATE=en_GB.UTF-8    
#>  [5] LC_MONETARY=en_GB.UTF-8    LC_MESSAGES=en_GB.UTF-8   
#>  [7] LC_PAPER=en_GB.UTF-8       LC_NAME=C                 
#>  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
#> [11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C       
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] ggplot2_3.3.3
#> 
#> loaded via a namespace (and not attached):
#>  [1] pillar_1.6.0      compiler_3.6.3    tools_3.6.3       digest_0.6.27    
#>  [5] evaluate_0.14     lifecycle_1.0.0   tibble_3.1.1      gtable_0.3.0     
#>  [9] pkgconfig_2.0.3   rlang_0.4.10      reprex_2.0.0      DBI_1.1.1        
#> [13] yaml_2.2.1        xfun_0.22         withr_2.4.2       styler_1.4.1     
#> [17] stringr_1.4.0     dplyr_1.0.5       knitr_1.32        generics_0.1.0   
#> [21] fs_1.5.0          vctrs_0.3.7       grid_3.6.3        tidyselect_1.1.0 
#> [25] glue_1.4.2        R6_2.5.0          fansi_0.4.2       rmarkdown_2.7    
#> [29] farver_2.1.0      purrr_0.3.4       magrittr_2.0.1    backports_1.2.1  
#> [33] scales_1.1.1      ellipsis_0.3.1    htmltools_0.5.1.1 assertthat_0.2.1 
#> [37] colorspace_2.0-0  labeling_0.4.2    utf8_1.2.1        stringi_1.5.3    
#> [41] munsell_0.5.0     crayon_1.4.1

^{Created on 2021-04-21 by the reprex package (v2.0.0)}

[teunbrand]

How the default minor breaks are calculated depends on the identity transformation in the scales package, which tries to make regularly spaced minor breaks, under the assumption of regularly spaced major breaks (I think).

library(ggplot2)
library(scales)
#> Warning: package 'scales' was built under R version 4.0.3

limits <- range(mpg$displ)
trans <- identity_trans()

# Based on scales::regular_minor_breaks()
trans$minor_breaks(c(2, 2.25, 4, 6), limits, 2)
#>  [1] 1.750 1.875 2.000 2.125 2.250 3.125 4.000 5.000 6.000 6.125 6.250

You can override how minor breaks are calculated, so I don't think ggplot2 is doing anything wrong here.

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  scale_x_continuous(
    breaks = c(2, 2.25, 5, 6),
    minor_breaks = breaks_width(1)
  ) +
  theme_bw()

^{Created on 2021-04-21 by the reprex package (v1.0.0)}