datraviz 2018 #27 - melting data

"melting" comes from the R community. it refers to taking a dataset with many columns, the "ice cube", and melting it into a dataset with many rows, the "icicle". implementation: nested forEach loops.

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d3.nest was replaced with d3.group and d3.rollup in v6.0 (src). so i'll be using v5.6.0 like the course example here.