I just worked through this issue myself. In my case, the stylesheet wasn't being properly referenced. I did try to import it at the top of my js file like so:
import '../../../../node_modules/react-vis/dist/style.css';
But, I later found that the styles only actually worked once I copied my style.css file from /node_modules/react-vis/dist into app/assets/stylesheets, where my application.css file has a require_tree statement that includes the style sheet in question.
This is a bit of a bootleg workaround, as importing the stylesheet from its home in /node_modules/react-vis/dist is probably best practice, but it worked for me.