enable annotations for grouped bar chart

I can't find any way of adding annotations to individual bars in grouped bar chart.

Minimal example: https://codepen.io/bleah1/pen/vYzMRWJ

• related https://github.com/plotly/plotly.py/issues/356 (textposition applies only to the text properties of each trace, annotations represent a layout property)

I am building a grouped bar chart with a slider and based on this workaround https://github.com/plotly/plotly.py/issues/356#issuecomment-235890265 I was able to add annotations to each frame.

首先复现链接 CodePen 中的最小分组柱状图，并查看相关的 plotly.py issue 及其基于 frames 的 workaround。完成标准是：分组柱状图中的单个柱可以接收 annotations，包括图表使用 slider 和 frames 的情况。