I've been investigating today about how to embed Plotaroute on Wordpress.com and thought that I'd explain my findings so far. Perhaps it will help other people in the future!
Note that this post applies only to blogs hosted on wordpress.com and not to any other WordPress sites or to self-hosted WordPress.
Wordpress.com doesn't allow the iframe html tag by default. So if you add the plotaroute share/embed html code, either in the Visual Editor or in the Code Editor, you get a blank map and only the small text underneath shows up ("route by someone on plotaroute.com"). You can see the documentation here listing which tags are stripped out: https://wordpress.com/support/code/
They say this is for security reasons on their shared hosting, and I have no reason to doubt that. But on higher plans, where they allow plugins (and presumably isolate your account to enable those) they also allow iframe tags to work too. This feature is only available on their new Pro plan (or legacy Business and eCommerce plans).
The final option would be to have Plotaroute added to their list of supported services for the Embed block - see https://wordpress.com/support/wordpress-editor/blocks/embed-block/ . I asked wordpress.com support to consider adding Plotaroute to that list, but I suspect it might not happen unless they receive lots of requests. There's only around 30 supported sites on that list at the time of writing. I'm not sure if Plotaroute can do anything to make it more likely (or easier) for Wordpress to add them to their approved list.
Anyway, I hope this information helps someone else in future, and I hope it might clarify why it works for some Wordpress users while not working for others!
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