By adding a fallback in the trace calendar getter in Histogram.crossTraceCalc so that when the Calendars module isn't registered (thus making _fullData[i].(x|y)calendar === undefined), we're still able to group traces in the correct bin groups.

fixes #4429

• before: https://jsfiddle.net/c8gfb6rp/2/
• after: https://jsfiddle.net/Lg5ucrew/
  • w/ https://gist.github.com/etpinard/ff8d940dfb831b54d41baa4e58f6af9d#file-plotly-cartesian-js
  • together with https://gitcdn.xyz/

Note also, that at present we only register calendars in the "main" plotly.js bundles:

cc @archmoj