Version 1.15.2 has been released as the MapQuest service no longer provides the free tile server that was the default configuration of GeoChart. The default has been switched to CartoDB.
However note that CartoDB currently allows up to 1 million map views per month, if you need more, you would have to subscribe to a paid plan (note that you can use any other tile server as well).
To mitigate this we are currently building our own tile server that would be governed by ZoomCharts Terms of Service, without relying on any third party.