title: geom_dotplot fill aesthetic changes data grouping
body: <p>Adding a fill variable to <code>geom_dotplot</code> introduces an unwanted grouping to the geom. Here's the behavior when you don't add a fill variable:</p>
  <pre><code>library(tidyverse)
  
  mtcars %&gt;%
   ggplot(
     aes(
       factor(vs), mpg
     )
   ) +
   geom_dotplot(
     stackdir = &quot;center&quot;, 
     binaxis = &quot;y&quot;, 
     fill = &quot;white&quot;
   ) +
   coord_flip()
  </code></pre>
  <p>returns</p>
  <p><a href="https://i.stack.imgur.com/jioHI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jioHI.png" alt="no fill dotplot" /></a></p>
  <p>I just want to color some dots, for attention purposes, but I don't want the dots to change location. If I try to affect the fill as follows:</p>
  <pre><code>mtcars %&gt;%
    rownames_to_column %&gt;% 
    mutate(
      merc = rowname %&gt;%
        str_detect(&quot;Merc &quot;) %&gt;%
        as.numeric %&gt;%
        factor
    ) %&gt;% 
    ggplot(
      aes(
        factor(vs), mpg, fill = merc
      )
    ) +
    geom_dotplot(
      stackdir = &quot;center&quot;, 
      binaxis = &quot;y&quot;, 
    ) +
    coord_flip() +
    theme(
      legend.position = &quot;none&quot;
    )
  </code></pre>
  <p>returns</p>
  <p><a href="https://i.stack.imgur.com/NGZ6Z.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NGZ6Z.png" alt="enter image description here" /></a></p>
  <p>I want the dots to stay where they are in the first plot, but introduce a fill aesthetic. Is this possible in <code>geom_dotplot</code>?</p>