Use of the Google Chart API is subject to a query limit of 50,000 queries per user per day. If you go over this 24-hour limit, the Chart API may stop working for you temporarily. If you continue to exceed this limit, your access to the Chart API may be blocked.
the request to the chart's url will be made by the client browser, which is a unique ip for practically every visitor. so, maybe that's 50,000 charts per day per visitor? unfortunately they don't really clarify what they mean, as they could be tracking the request referrer.
Usage Policy
Use of the Google Chart API is subject to a query limit of 50,000 queries per user per day. If you go over this 24-hour limit, the Chart API may stop working for you temporarily. If you continue to exceed this limit, your access to the Chart API may be blocked.