Is there a DGML Viewer?

There is a DGML Viewer in Visual Studio 2019. You can install it via Visual Studio Installer. Just open Visual Studio Installer, click on Modify, select Individual Components, Code Tools, DGML Editor. To create the DGML files from a DbContext on an EF Core application, you can use EF Core Power Tools. The image below is from a DGML generated by EF Core Power Tools and viewed using DGML Editor. I hope it helps.

I encourage you to submit a request by selecting Suggest a feature on this page as further support for a standalone DGML viewer. It might help persuade the product team that there's enough of a demand for one.

The DGML Editor was not available in Visual Studio 2017 by default so I could not see DGML files visually and only saw them in the xml format. I had to fix this by explicitly installing the DGML editor. Please install it as such: (Tools > Get tools and features... > Individual Component > Code tools > DGML Editor)