Here’s how to get the “Insert Diagram” (Diagram Editor) plugin of Zim working under Windows. First install graphviz. Second, set the path using the GUI: in the start menu, go to Control Panel > System > Advanced System Settings (on the left) > in the Advanced tab choose Environmental Variables at the bottom. There should be a variable called Path at the top; modify it by adding ;C:\Program Files\Graphviz2.38\bin\ at the end (it may be easier to copy the text and add the bit in a text editor; obviously modify to match your version number). (while there, you may also modify the lang variable at the bottom if the journal plugin gives you a different calendar from what you’re used to: try en_US or en_GB instead of en). Finally restart Zim, and you can now enable the plugin.