Many critical decisions for individuals and organizations are often framed as preferential choices: the process of selecting the best option out of a set of alternatives. This paper presents a task-based empirical evaluation of ValueCharts, a set of interactive visualization techniques to support preferential choice. The design of our study is grounded in a comprehensive task model and we measure both task performance and insights. In the experiment, we not only tested the overall usefulness and effectiveness of ValueCharts, but we also assessed the differences between two versions of ValueCharts, a horizontal and a vertical one. The outcome of our study is that ValueCharts seem very effective in supporting preferential choice and the vertical version appears to be more effective than the horizontal one. Categories and Subject Descriptors H.5.2 [Information Interfaces and Presentation]: User Interfaces--Graphical user interfaces (GUI); I.3.6 [Computer Graphics]: Methodologies and Tech...