dydx() with change = "iqr" not working as expected

[X] a possible bug
[ ] a question about package functionality
[ ] a suggested code or documentation change, improvement to the code, or feature request

I think option change = "iqr" in function dydx() does not work properly. For example:

library(margins)
fm <- lm(mpg ~ hp, data = mtcars)
dydx(mtcars, fm, "hp", change = "iqr")
#>      dydx_hp
#> 1  -5.731175
#> 2  -5.731175
#> 3  -5.731175
#> 4  -5.731175
#> 5  -5.731175
#> 6  -5.731175
#> 7  -5.731175
#> 8  -5.731175
#> 9  -5.731175
#> 10 -5.731175
#> 11 -5.731175
#> 12 -5.731175
#> 13 -5.731175
#> 14 -5.731175
#> 15 -5.731175
#> 16 -5.731175
#> 17 -5.731175
#> 18 -5.731175
#> 19 -5.731175
#> 20 -5.731175
#> 21 -5.731175
#> 22 -5.731175
#> 23 -5.731175
#> 24 -5.731175
#> 25 -5.731175
#> 26 -5.731175
#> 27 -5.731175
#> 28 -5.731175
#> 29 -5.731175
#> 30 -5.731175
#> 31 -5.731175
#> 32 -5.731175
# I would have expected the same result as in
dydx(mtcars, fm, "hp", change = c(quantile(mtcars$hp, 1/4), quantile(mtcars$hp, 3/4)))
#>      dydx_hp
#> 1  -5.697061
#> 2  -5.697061
#> 3  -5.697061
#> 4  -5.697061
#> 5  -5.697061
#> 6  -5.697061
#> 7  -5.697061
#> 8  -5.697061
#> 9  -5.697061
#> 10 -5.697061
#> 11 -5.697061
#> 12 -5.697061
#> 13 -5.697061
#> 14 -5.697061
#> 15 -5.697061
#> 16 -5.697061
#> 17 -5.697061
#> 18 -5.697061
#> 19 -5.697061
#> 20 -5.697061
#> 21 -5.697061
#> 22 -5.697061
#> 23 -5.697061
#> 24 -5.697061
#> 25 -5.697061
#> 26 -5.697061
#> 27 -5.697061
#> 28 -5.697061
#> 29 -5.697061
#> 30 -5.697061
#> 31 -5.697061
#> 32 -5.697061

^{Created on 2020-06-21 by the reprex package (v0.3.0)}

Session info

``` r sessionInfo() #> R version 4.0.0 (2020-04-24) #> Platform: x86_64-apple-darwin17.0 (64-bit) #> Running under: macOS High Sierra 10.13.6 #> #> Matrix products: default #> BLAS: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRblas.dylib #> LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib #> #> locale: #> [1] de_DE.UTF-8/de_DE.UTF-8/de_DE.UTF-8/C/de_DE.UTF-8/de_DE.UTF-8 #> #> attached base packages: #> [1] stats graphics grDevices utils datasets methods base #> #> other attached packages: #> [1] margins_0.3.23 #> #> loaded via a namespace (and not attached): #> [1] Rcpp_1.0.4.6 digest_0.6.25 MASS_7.3-51.6 magrittr_1.5 #> [5] evaluate_0.14 highr_0.8 rlang_0.4.6 stringi_1.4.6 #> [9] data.table_1.12.8 rmarkdown_2.1 prediction_0.3.14 tools_4.0.0 #> [13] stringr_1.4.0 xfun_0.14 yaml_2.2.1 compiler_4.0.0 #> [17] htmltools_0.4.0 knitr_1.28 ```

leeper / margins

dydx() with change = "iqr" not working as expected #151