Changing cplot.lm to cplot.default would let objects of an arbitrary S3 class that look like an lm object to take advantage of this great function without requiring users to use margins:::cplot.lm(obj).

This would be super helpful unless there's a justification for keeping it as cplot.lm!

Here's a MWE for estimatr:

devtools::install_github("DeclareDesign/estimatr")
lmres <- lm_robust(mpg ~ hp * wt, data = mtcars)
class(lmres)
#> [1] "lm_robust"
margins::margins(lmres, vce = "delta") # looks great!
margins:::cplot.lm(lmres, x = "hp", dx = "wt", what = "effect") # also looks great!
margins:::cplot.lm(lmres, x = "hp") # also looks great!