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:

未指定檔案、測試或進入點。定位 splom 軌跡處理，然後檢查 dragmode 的預設值和取消選取點的樣式是如何選擇的；在變更這兩個預設值之前，比較現有的選取行為。完成的標準是 splom 以 select mode 啟動，且選取的點在密集面板中仍然能夠清楚區分。