selectize.js default behavior: remove label and allow for placeholder

从 R 的 highlight() 入口点以及提供的 R Markdown 重现所触发的 selectize.js 行为开始。验证多个 plotly 图形如何共享选择状态，然后使标签抑制和占位符文本可配置，同时保持各图形之间的选择彼此独立。

I would like to create an R Markdown document with > 1 plotly figure where points can be highlighted with the selectize.js widget. Here a reproducible example

---
title: Test
output: html_document
---

# wt vs mpg

```{r, message=FALSE, warning=FALSE}
library(plotly)
myData <- highlight_key(mtcars, ~rownames(mtcars))
myFig <- plot_ly(myData, x = ~wt, y = ~mpg) 
highlight(myFig, selectize = TRUE)
```

# disp vs mpg

```{r, message=FALSE}
myData <- highlight_key(mtcars, ~rownames(mtcars))
myFig <- plot_ly(myData, x = ~disp, y = ~mpg) 
highlight(myFig, selectize = TRUE)
```

Two Design improvements would be nice

1. No label above selectize widget
2. Placeholder text in the <input> field of the widget

For the first problem I found a workaround using highlight_key(..., group = " "). However I need to define for each plot a new "invisible" group identifier otherwise the highlighting of the plots will be linked (i.e. if I highlight a car in one plot it will be highlighted in all plots). At the moment I use group = " ", group = " ", group = " ", and so on but this is stupid.

A workaround for the placeholder text was already described on Stack Overflow. But it does not work out of the box if there are multiple plotly figures in one R Markdown document.

Optimal solution would be to have two additional arguments in the highlight() function

• label = NULL makes sure that no label is plotted
• placeholder = "" to define the placeholder in the selectize box.

Let me know what you think of it : )