Presently, the best way to identify the creator of a dashboard or graph is to run an audit query. For example,
Please note that full user permissions are required to review the audit logs, and some accounts may restrict this information to a subset of users.
A feature request has been raised which will enable user information to be stored with the dashboard. Presently, only the dashboard's creation and last modified dates are retained (access the dashboard via the "Dashboards" option, then click "..." -> "Edit JSON" to review):