My issue with online APIs like this is that you can't guarantee that this will always be around (knowing Google's penchant for clearing up little-used projects), or choose the version that you know works (unlike jquery...)
I have a feeling that my own project will break after a while and I'll need to change some API calls or move to the new Material charts, or a new chart API altogether...
This is a nice quick and easy API, but I too have similar long-term concerns...