GeoChart uses Leaflet.js v7.7.0. That doesn't offer Frictional Zoom Level and works with Zoom Level as integer.
Starting from Leaflet.js v1.0.0 it is possible to use Frictional Zoom Level. If you really wish you can download other Leaflet.js version and replace v.7.7.0 with newer one on your own risk as ZoomCharts currently supports only Leaftet.js v7.7.0 and no newer version is supported.
More information about Leaflet zoom levels: