The lines are svg paths that are rendered by the browser. Embedded in the path is the color which is most likely contained somewhere in the JIRA code. There is no setting that I know of that will allow you to change the color and custom CSS will not work as there is no attribute other than path to tie it to.
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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