MissingKeyMapError that just began occurring with Google Charts + displayMode = text

Same problem as described in ERROR: Google Maps API error: MissingKeyMapError, except that:

1. this is with Google Charts instead of Google Maps
2. unlike for Google Maps, there seems to be no documentation on how to get a key or use a key when it comes to charts / visualization (e.g. var map = new google.visualization.GeoChart(...)Googling "google charts MissingKeyMapError" yields NOTHING.
3. this error does not seem to occur if the html+js is served from a web server ruing on localhost. It only occurs when that html+js is saved to file and then the file is opened in a browser
4. this error only seems to occur when setting displayMode = 'text' (maybe in doing so Charts is calling Maps and I need to somehow pass some Maps API key inside there?)

This problem did not occur a couple of days ago. Googling "MissingKeyMapError charts" yields NOTHING.

Here's the code snipped that seems to recreate the error: var map = new google.visualization.GeoChart(document.getElementById('map2-container')), map_options = { region: 'US', resolution: 'provinces', displayMode: 'text', };

    map.draw(stateText, map_options);