Zeptal jsem se googla a dozvedel se:

Usage Policy

There's no limit to the number of calls per day you can make to the Google Chart API. However, we reserve the right to block any use that we regard as abusive, an apparent denial of service attempt for example. If you think your service will make more than 250,000 API calls per day, please let us know by mailing an estimate to chart-api-notifications@google.com.