One cell of Visualize.Components.Board (spec/15 §9.1): placed on the grid, one tab
stop carrying data-vis-cell, its chart wrapped in CrosshairHook, BrushHook and
KeyboardHook, and its state drawn over or instead of the chart. Updated by
Visualize.Components.Board.forward/2 with a Visualize.Board.View; never blank, a
cell with no view yet shows :loading.
Its chart's KeyboardHook pushes pick here (spec/15 §13): the board answers the
row behind the point, and the cell opens its drill target through the page, or
selects the datum's identity across the board.
Its chart's TooltipHook pushes a mark's data-tooltip-event here too (D-161), and
the cell sends it to the page as {:visualize_board_event, board_id, %{cell:, event:, fields:}}. Every event other than pick is a mark's.