color bars by the first group

[mccalluc]

• Fix Better visualization options for groups? #245
• Remove the cutoff kwarg which we never used
• Color bars by the first group in the compound key
• Instead of joining with a space, join with a semi-colon: If the key itself happened to include a semicolon, some colors might be reused. This isn't a great practice, but the downsides are small.

The first key is not necessarily the most meaningful key, but I don't want to get bogged down in graph configuration.

Not necessarily a bug, but definitely a quirk: the order of keys is different when they are inferred, versus when they are supplied explicitly. For example with sample.csv,

• if groups are not provided, it is grade bin, and then class year.
• if class year groups are provided, it is class year, and then grade bin.