I have the same problem in 3.4.6 – this is on WP 4.9.15 and PHP 7.3.
Previously created visualizations that are already embedded in a page still work – but they don’t appear in the library. I’ve checked the server logs and there aren’t any errors. I fixed it by reinstalling the previous version of the plugin – 3.4.5.
In 3.4.6 it looks to me like the visualizations were just hidden in the Library admin screen. When I look in the browser developer mode I can see that they’re in the html of the page – but hidden with css.
The “.visualizer-chart” class is still set to “display:none” on the library page. In version 3.4.5 it sets them to be “display:block” using inline styles – I assume a javascript file sets the visibility once the library page loads.
However when I tried to change the css style to be visible they didn’t quite load properly – and the chart preview images didn’t show up. I can only assume there’s some kind of of javascript error – but I didn’t see one in the browser developer console.