title: Setting Heat Map Color range as per given Sequence in hcharter
body: <p>Here I am trying to create a heat map using <code>highcharter::hcharter()</code> where <strong>less than -1</strong> should be one color (pink), <strong>-1 to 1</strong> should be transparent or white color, and <strong>greater than +1</strong> should be another color (violet).  </p>
  
  <p>Presently I have written code using <code>hchart()</code> and used <code>color_stops</code> to change the color format. But this works really well when data is centered to 0, but when it isn't centered to 0 let say from <strong>-5 to +7.5</strong> white/transparent color would be shifted to 1 as shown in the image below.  </p>
  
  <pre><code>a1 &lt;- rnorm(30,0,2)  
  a2 &lt;- rnorm(30,0,2)  
  a3 &lt;- rnorm(30,0,2)  
  a4 &lt;- rnorm(30,0,2)  
  a &lt;- cbind(a1,a2,a3,a4)
  heatmap_matrix &lt;- as.matrix(a)
  
  library(highcharter) 
  hchart(heatmap_matrix) %&gt;%  
    hc_colorAxis(stops = color_stops(n = 3, colors = c("#FF1493", "white", "#800080")))
  </code></pre>
  
  <p>For data range between <code>-5 to +7.5</code><br>
  -5  to -1 should show pink gradient color<br>
  -1 to +1 should show white color<br>
  +1 to 7.5 should show violet gradient color   </p>
  
  <p><a href="https://i.stack.imgur.com/Kr51j.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Kr51j.jpg" alt="Heat Map sample plot using hcharter"></a>    </p>