  Customization is crucial for making visualizations accessible to blind and
low-vision (BLV) people with widely-varying needs. But what makes for usable or
useful customization? We identify four design goals for how BLV people should
be able to customize screen-reader-accessible visualizations: presence, or what
content is included; verbosity, or how concisely content is presented;
ordering, or how content is sequenced; and, duration, or how long
customizations are active. To meet these goals, we model a customization as a
sequence of content tokens, each with a set of adjustable properties. We
instantiate our model by extending Olli, an open-source accessible
visualization toolkit, with a settings menu and command box for persistent and
ephemeral customization respectively. Through a study with 13 BLV participants,
we find that customization increases the ease of identifying and remembering
information. However, customization also introduces additional complexity,
making it more helpful for users familiar with similar tools.
