Offering end users, experts on the domain, means to explore and query data stored in the information systems they work with can generate large benefits for organizations and society. Frequently, expressing ad-hoc queries with a query language is complex for end users, even when they use visual languages or graphical interfaces. GraphTQL a new visual query language for data graphs that aims to facilitate end-users formulating complex queries was developed. The design process of GraphtTQL followed the User-Centered Design Approach (UCD). The results of interviews and usability tests with end users are presented.