It looks like it is a "cache buster". So "salt" is a bit of a misnomer -IMO-, as it is usually used in a cryptographic context.
In this case, the "salt" (i.e. cache buster) prevents the browser (or a proxy) from caching the graph/data, because the URL is always unique. Because of this we can be sure that the browser/proxy is always requesting graphs of the server (at the cost of additional round trips when perhaps the aren't needed -because the data may not have changed-).