… NOT MERGE- tabindex attribute on marks- focusin/focusout event handling- Adds support for focus(element) event method, so that one can move focus to SVGElements within the scenegraph.Note that in order to tab in and out of the Vega view, we must allow keydown events in the Vega config.

… false for marksNo tabindex attribute should be included on marks where interactive === false.

Add WIP stacked bar chart from Data Navigator to see if we can reproduce the functionality using focus management in Vega.

…istencyDifferent browsers seem to render focus outlines for elements within an svg inconsistently. The new Interactive Stacked Bar Chart Example demonstrates a focus outline rendered within Vega, using SVG. The outline property allows a user to set the outline style for a mark or group, for example "outline: none."

Adds tabindex="-1" to ownerSVGElement when it contains other focusable elements, so that the SVG element itself does not become an additional tab stop.

Fixes issue in Interactive Stacked Bar Chart example where the focus rectangle remained visible, surrounding a point with no width or height, when the user tabs away from the chart.

Adds ability to set role="application"  to the view so that windows based screen readers switch into "forms mode" or "application mode" when entering an interactive visualization.

Adds interactive line chart example that demonstrates managing focus between lines and points, and displaying tooltips for points.

…-a11y