  We investigate direct manipulation of graphical encodings as a method for
interacting with visualizations. There is an increasing interest in developing
visualization tools that enable users to perform operations by directly
manipulating graphical encodings rather than external widgets such as
checkboxes and sliders. Designers of such tools must decide which direct
manipulation operations should be supported, and identify how each operation
can be invoked. However, we lack empirical guidelines for how people convey
their intended operations using direct manipulation of graphical encodings. We
address this issue by conducting a qualitative study that examines how
participants perform 15 operations using direct manipulation of standard
graphical encodings. From this study, we 1) identify a list of strategies
people employ to perform each operation, 2) observe commonalities in strategies
across operations, and 3) derive implications to help designers leverage direct
manipulation of graphical encoding as a method for user interaction.
