title: R highcharter mapline color does not change
body: <p>No matter what I try I have not been able to get the maplines (pointing out to the boxes on the right of the image) to change colors.
  <a href="https://i.stack.imgur.com/wcVrP.png" rel="nofollow noreferrer">see plot here</a></p>
  
  <p>The default is a light grey that is barely visible (see pic). Here is a java script example of the highcharts template I am using where the maplines are silver:</p>
  
  <p><a href="https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/maps/demo/us-data-labels/" rel="nofollow noreferrer">https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/maps/demo/us-data-labels/</a></p>
  
  <p>Update: by adding a data.frame with the lines and the colors I want them to have I can get one line to be colored, but not the others.</p>
  
  <p><a href="https://i.stack.imgur.com/R8FLn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/R8FLn.png" alt="plot with one colored line"></a></p>
  
  <pre><code>library(highcharter)
  library(usmap)
  library(dplyr)
  
  ##### get data #######
  df &lt;- usmap::statepop
  
  us_small &lt;- download_map_data("countries/us/custom/us-small")
  
  leader_lines &lt;- data.frame(color = rep(hex_to_rgba('#000000'),9),
                             leader_lines = rep('__separator_lines__',9))
  
  ##### plot #######
  highchart() %&gt;% 
    hc_add_series_map(us_small, df,
                      value = "pop_2015", joinBy = c("woe-name", "full")) %&gt;%
    hc_add_series_map(us_small, 
                      leader_lines, 
                      value = 'color', 
                      joinBy = c("hc-group", "leader_lines"), 
                      lineWidth = 2,
                      type = "mapline")
  </code></pre>