when z is not defined, zorder crashes

[Fil]

this crashes:

Plot.text(versions, Plot.stackX({x: "downloads", order: Plot.valueof(versions, "version")})).plot();

stack.js?t=1672927922568:329 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'map')
    at orderZDomain (stack.js?t=1672927922568:329:12)
    at stack.js?t=1672927922568:321:29
    at Text.transform (stack.js?t=1672927922568:86:26)

this works:

Plot.text(versions, Plot.stackX({x: "downloads", z: [], order: Plot.valueof(versions, "version")})).plot();

see https://observablehq.com/@mbostock/npm-daily-downloads#comment-87022bafaa591b9a