`hoveron: 'fills'` does not work for stacked area chart

Using hoveron: 'fills' with a stacked area chart doesn't result in hover tooltips being shown when hovering over the filled in areas.

Plotly.newPlot('test', [
	{x: [1,2,3], y: [2,1,4], stackgroup: 'one', hoveron: 'fills', name: "a"},
	{x: [1,2,3], y: [1,1,2], stackgroup: 'one', hoveron: 'fills', name: "b"},
]);

Here's a JSFiddle illustrating the issue - hovering over the blue or orange filled in areas has no effect.

使用 JavaScript 程式碼片段或連結的 JSFiddle 重現問題，然後追蹤堆疊面積圖對 hoveron: 'fills' 的懸停處理。完成的判定標準是：將游標移到每個已填滿的堆疊區域上時，都會顯示預期的工具提示，且不改變其他懸停模式的行為。