add selectable shapes or absolute marker sizes

At first I started with shapes since their size can be explicitly defined in the same domain as the data (i.e. not an arbitrary number that doesn't correspond with the data as with marker size attribute). However, the problem that arises is that shapes do not express any visual feedback when selected (in selectData) as markers do.

So, unfortunately, I've run into a limitation with each method (shapes don't provide feedback when selected nor are they selectable [however you could put a transparent marker in the center which I did for awhile] and marker sizes/shapes cannot be defined in the same domain as the data). Alternatively, I think it might be useful to explicitly define the size and shape of a marker in the same "domain" as the data so that if you were to zoom in or out, the dimensions of the marker do not scale but instead maintain the specified dimensions because its size and shape is important relative to the data. Something like #2193 except for markers and no need for the movable functionality.

Additionally, this would provide some more freedom in customizing the marker i.e. the marker could be defined by an aggregate of shape objects (which is different than #330). That is, if it is important that a data point be represented by a red square behind a blue circle each of a specific size - it could be done.

首先跟踪 plotly.js 中的 selectData 以及现有的 marker 和 shape 选择行为。比较所要求的替代方案：可选择的 shape、基于数据域的 marker 大小，以及复合 marker 自定义。应将完成定义为一种具体的受支持行为，具有可视化的选择反馈，并且其尺寸在缩放期间仍然具有意义。