We would like to be able to check the values of parameters or somehow introspect the layer's params. For e.g. the geom_hist accepts a binwidth , and we'd like to check that they are using the correct value there. Another example is checking if a geom_boxplot is using a specific outlier.alpha (e.g. 0.01).

We would also like to be able check a layer created using a stat_ function, for example the stat_bin for creating a geom_bar where we can also grade the parameters like bins.