possible naming collisions

[rpruim]

Currently, size and alpha (and a few other things) are arguments for mod_plot(). This means that using those as variable names in the model will preclude using ... to set their values.

Going forward, if we want to allow things like size = ~var1 (to map size to var1) but also possibly size = 7 (to set the value of a variable in the model), then we have an ambiguity. A similar things happens in gf_dist(), see ProjectMOSAIC/ggformula#74 for one possible solution. That solution would be to use plot_size for the plot property when size is a variable in the model.