I think it's important to highlight (here or in the API) that all the series should have the same number of elements. Otherwise, you get some pretty unintuitive plots. For example, if I have a row every time the amount of fruit changes: ``` [ {fruit: "banana", day: 1, stock: 20}, {fruit: "banana", day: 3, stock: 15}, {fruit: "apple", day: 1, stock: 10}, {fruit: "apple", day: 2, stock: 25}, ] ``` It took me a while to realize I needed to add the values for the missing days to get the proper plot: ``` [ {fruit: "banana", day: 1, stock: 20}, {fruit: "banana", day: 2, stock: 20}, // add this {fruit: "banana", day: 3, stock: 15}, {fruit: "apple", day: 1, stock: 10}, {fruit: "apple", day: 2, stock: 25}, {fruit: "apple", day: 3, stock: 25}, // add this ] ```

This is important for area charts, but not necessarily for the stack transform; for instance the chart at https://observablehq.com/@observablehq/plot-stack#congress doesn't have such a requirement.