Layer Cake
annotations

Click a chart to add an annotation. Drag it to move it and double-click to edit its text. Hover it to show its handles: drag a circle outward to draw an arrow, or drag the diamond to move its anchor point. Cmd+click (Ctrl+click on Windows and Linux) an arrow's control point to change its swoopiness.

Changes are logged to the browser console. You can paste that config object into your own chart.