Backend: refactor optional binning in lineplot and the bin* family of fxns

Start by locating the lineplot, bin* and group* functions in the R package and compare their current arguments and behavior for numeric and date axes. Trace how unbinned results are represented, then identify the relevant tests or examples. Done means the optional binning design and the proposed bin* to group* refactor are implemented consistently without relying on placeholder bin widths.

so currently if you dont want the line plot binned youd have to pass it a binwidth of 0 for numeric x axes and 1 day for dates. the response you get is consistent w if the data were to have been binned, except bin start and bin end are the same value and the bin label is essentially useless.. that seems less than ideal.

additionally, the bin* family of fxns now have some members that take only an x-axis arg, and some that take both an x and y axis. that also seems less than ideal, and while on the surface unrelated to the prev problem im starting to suspect the solution to both problems might be the same..

i think to solve both these problems we should consider adding the concept of 'bin' optionally to any/all group* fxns where its even remotely relevant. if we can we should also refactor the two bin* fxns that only take an x-axis argument to take both x and y args and become group* fxns themselves. this is likely to become a bit involved and requires some careful thinking, so i didnt want to try to attach it to the lineplot deadline.