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After saving the Mermaid code, the rendered version of the diagram is shown. On this view, you can click the download icon to download the diagram as PNG image.
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The button “Add to description” saves the diagram as image as ticket attachment and adds this image to the ticket’s description. Please note: The generated image is a "snapshot" of the diagram, i.e. after updating the Mermaid code, you have to click the button again and remove the old image from the description if you want to reflect the Mermaid diagram updates also in the ticket's description.