The Accessibility panel is located in the Customize->Basic accordion menu in the table editor. The options presented let you toggle Accessibility mode for the chart and to enter a description in a textfield.
Enable Accessibility: On and checked means that accessibility support has been enabled for the chart. Off and unchecked means that accessibility support has not been enabled for the chart.
Text Description: Enter a textual description of the chart to be stored in the chart’s screen reader information region.