Though Portals had some built-in features around dashboards and public-facing data visibility, we often encountered issues with scalability and user management. When we created Murano, we did so in order to make device data as flexible as possible so you can present it to anyone you want, but (at the moment) it requires a bit of a heavier lift on the side of the developer (you).
The current paradigm for creating public dashboards is to create a Solution > Application, which is effectively a hosted web application. The link takes you to a tutorial that sets your free account up with a full-fledged web application complete with control of the device from the app, user management and DNS routing.
What you're asking for (public dashboard) is a special, more simple case of hosted web applications, where the goal is to make data accessible from anywhere by anyone. We're working on a freeboard solution to this problem. The way (we hope) it will work is that you will be able to launch a public dashboard using a customized freeboard application using either the Murano Exchange or the Murano CLI tool.
Thanks for your post!