OptionalgetReturns the chart layout interface for accessing element positions.
Optional, because a great many packages do not have one: the material
builders (google.charts.Bar, google.charts.Line), and Calendar,
Gauge, Sankey, OrgChart and TreeMap among the classic ones. On those a
call throws chart.getChartLayoutInterface is not a function, so it is
asked through chartLayout() rather than called directly -- a throw here
travels out of the conversion inside the caller's own ready handler and
costs the chart its reading altogether, where the intended degradation is
no highlight with the audio, text and braille intact.
Common interface shared by all Google visualization chart types.