A design applied to its data (spec/14 §7.3, §14.10).

Visualize.Chart.apply/2 is the only constructor: chart is the concrete %Visualize.Chart{} — no variable anywhere in it, vars empty, the slot declarations kept as the data contract; sources are the bound sources by slot name, each as the pool gave it; frame is the %Visualize.Chart.Frame{} realised over them, whose Visualize.Chart.Frame.scales/1 is the escape hatch. Visualize.Chart.render/2 draws it. The struct has no map or JSON form: it holds the data the design was applied to, and a design, its fragments and a binding are what is stored (D-65).