--- a/eric6/Preferences/__init__.py	Wed Oct 23 20:06:12 2019 +0200
+++ b/eric6/Preferences/__init__.py	Thu Oct 24 19:30:01 2019 +0200
@@ -479,6 +479,7 @@
         "PreviewMarkdownNLtoBR": True,
         "PreviewMarkdownUsePyMdownExtensions": True,
         "PreviewMarkdownMathJax": True,
+        "PreviewMarkdownMermaid": True,
         "PreviewMarkdownHTMLFormat": "HTML5",  # XHTML1, HTML4, HTML5
         "PreviewRestDocutilsHTMLFormat": "HTML5",   # HTML4, HTML5