Keyboard helpers

enableKeyboardNavigation

Lets a keyboard move through a chart's data points: the svg takes one tab stop, the arrow keys move between points (up and down by a row on a grid), Home and End go to the first and last, and Escape, or leaving the chart, lets go. Each move fires the mouse events a pointer would, so the chart's own hover behaviour, its custom events and the tooltip follow focus.

Run on every render: it keeps one set of listeners per svg, and reads the stops on each key press, so they follow the data.

Kind: global constant