Xtykutl

This is half a question, as I have this 'sort of' thought through. Still, I'd like to have some confirmation. Here it goes:

From what I've seen so far, a group holds all the information necessary for a plot. Let's imagine a bar chart for the 'canonical' dc data array. We define a dimension on type and then a group. The group data will give us all the necessary coordinates for drawing the bars.

Why do we need dimensions, then? Is this for plotting, or just for keeping track of the filters and dynamically updating the chart?

This is only half an answer. :)

Yes, there is some redundancy between dimensions and groups for sure.

1. The group key function needs to be a refinement of (and must be consistent with) the dimension key function.
   There's only one place where I've found it's helpful to refine the group key function more than the dimension key function: when the dimension is time and the group is some quantization of time like months or hours. Otherwise there is no need to specify the group key function at all. I haven't seen too many people create sets of time-series charts where different charts are quantized at different levels, so I am not sure if that's the motivation.




2. Filtering happens through the dimension, not the group, and a group does not observe its dimension's filters - so you might want them to be different objects so you that you can optionally have the chart respond to its own filters. That's pretty rare, though - usually a chart will read data from a group on the dimension it filters.

Any other reasons? Please add to this list!