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GANTT: set default color for data without index value

When creating a GANTT chart, we have set the index (for coloring) to a column with categorical values. In some cases, the categorical column is empty (NaN) and the GANTT bar color is white:

How can I configure the GANTT to use a different default color, so the bars are visible?

Gantt charts aren’t part of the plotly.js official API. I suspect there might be a plotly.py wrapper for them, but would you mind telling us how you created those?

Jep, it is a Python wrapper: