Allow for custom data for discrete scales

[willgearty]

There should be some way to have a discrete scale where the time bins are not 1:1 with the discrete axis bins (e.g., if you want both periods and stages on the axis). One solution for this would be to add an option/argument for coord_geo() that prevents the discrete conversion of the data (92d9ad2). This way, a user could provide their own data that is already set up for a discrete scale. Obviously, this argument would need to be able to be a list like other arguments, in case the user wants one scale to be converted to discrete and another not converted.

[willgearty]

Another possible solution would be to allow for a dataset where there are multiple rows with the same abbr but with different names, and there could be an option to merge time bins with the same abbr.