Better selection highlighting for splom traces

Crossfiltering within a splom is perhaps its most powerful (and fun!) feature. We should make this better by:

• When there's a splom trace, make dragmode: 'select' the default, rather than 'zoom', so more people are encouraged to find and use this!
• Further fade the deselected points. splom traces encourage many small panes, which makes points overlap a lot, so the opacity we normally use for deselection builds up to the point you can't distinguish selected from deselected very well. We should decrease the opacity, and/or partially desaturate the colors, by default. Here's an example from https://plot.ly/python/splom/ - off-diagonal you can certainly see the selection, but it doesn't really pop out as much as it could. On diagonal, where the overlap is much greater, you can't even really see the selection:

未指定文件、测试或入口点。定位 splom 轨迹处理，然后检查 dragmode 的默认值和取消选择点的样式是如何选择的；在更改这两个默认值之前，比较现有的选择行为。完成的标准是 splom 以 select mode 启动，并且选中的点在密集面板中仍然能够清晰区分。