A chart widget can modify its chart using
// Add a horizontal line at 1.15 onto the widget's chart (with default colours; unspecified)
Framework.ChartChange({
mode: "active", // In a chart widget, mode:active always refers to the widget's own chart
command: "create-drawing",
drawing: {
type: "horizontalLine",
points: [{value: 1.15}]
}
});