Hi @logman , first this isn't a burndown chart it appears to be a Version report.
In this report there are a number of things being charted. The blue line will not be linear I think as it is driven by actual work and will vary as to predict the actual completion date. At least I believe this is the case.
Here is an article that might be helpful - Version-Report
Learn how to use two new reports for next-gen projects in Jira Cloud: Cumulative flow diagram and Sprint burndown chart. Ivan Teong, Product Manager, Jira Software, demos the Cumulative ...
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