Calculate unique values for ordinal scales

[mhkeller]

Solves #27

[mhkeller]

The only weird thing in this is I modified the flatten function to pass string keys on arrays onto the child object. This could cause issues such as key globbering or other object distortions and I may not include it. The reasoning for this for the moment is there is no other way to detect series names when using a wide data structure in stack layouts. This way, it copies the key field from the stack group array onto each child array, which is what is passed to the calcUniques function with flatData as the argument.

[mhkeller]

I got rid of the weird stuff in flatten. In all of those stacked examples, you compute the domain list manually anyway so there's no extra work except setting it as a prop. if the user wanted to, they could loop through the stacked data and add their own value on the z key