A growing number of visualization tools are now publicly released on the Web. While this has many benefits, such as reaching more users without any installation time or procedure, it is often unclear how those tools are being used. The most common method to remotely observe usage is remote logging through a web server. Analyzing recorded logs has already been successful to improve the usability of tools, assess the performance of users and even to enrich the user interface with histories or logs visualizations. However, from our own practice of recording and analyzing logs, we have found a lack of methodology to support this process and use the results consistently. This workshop aims at bringing the visualization community together to share their practice on logging mechanisms, methods, logs reporting, analysis, visualization, and the underlying infrastructure to support logging on the server side and on the client side. Expected benefits include raising awareness on the potentials of logging for visualization, providing shared tools and methods to instrument visualizations, show how logging can improve visualization and interaction techniques, and scale to large number of users and sessions.