  The ubiquity and on-the-go availability of mobile devices makes them central
to many tasks such as interpersonal communication and media consumption.
However, despite the potential of mobile devices for on-demand exploratory data
visualization, existing mobile interactions are difficult, often using highly
custom interactions, complex gestures, or multi-modal input. We synthesize
limitations from the literature and outline four motivating principles for
improved mobile interaction: leverage ubiquitous modalities, prioritize
discoverability, enable rapid in-context data exploration, and promote graceful
recovery. We then contribute thirteen interaction candidates and conduct a
formative study with twelve participants who experienced our interactions in a
testbed prototype. Based on these interviews, we discuss design considerations
and tradeoffs from four main themes: precise and rapid inspection, focused
navigation, single-touch and fixed orientation interaction, and judicious use
of motion.
