`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 代码片段或链接的 JSFiddle 重现该问题，然后跟踪堆叠面积图针对 hoveron: 'fills' 的悬停处理逻辑。完成标准是：悬停在每个已填充的堆叠区域上时都会显示预期的工具提示，同时不改变其他悬停模式的行为。