Fix View diagram : Too many agent to display (#1664) (#1935)

* Fix View diagram : Too many agent to display (#1664)
Making a GET request is simple, but GET URLs are limited to 2K characters.
The Google Chart API supports HTTP POST for chart requests up to 16K long. (*1)

(*1) https://developers.google.com/chart/image/docs/post_requests

* Fix View diagram : Too many agent to display (#1664)

Use  Faraday to POST api. 
Use when case to check response.status

app/helpers/dot_helper.rb

@@ -8,14 +8,23 @@ def render_agents_diagram(agents, layout: nil)
         } rescue false)
       decorate_svg(svg, agents).html_safe
     else
-      uriquery = URI.encode_www_form(cht: 'gv', chl: agents_dot(agents))
-      #Get query maximum length should be under 2048 bytes with including "chart?" of google chart request url
-      if uriquery.length > 2042
-        "Too many agent to display, please check unused agents"
+      # Google chart request url 
+      faraday = Faraday.new { |builder|
+        builder.request :url_encoded
+        builder.adapter Faraday.default_adapter
+      }
+      response = faraday.post('https://chart.googleapis.com/chart', { cht: 'gv', chl: agents_dot(agents) })
+
+      case response.status
+      when 200
+        # Display Base64-Encoded images
+        tag('img', src: 'data:image/jpg;base64,'+Base64.encode64(response.body))
+      when 400
+        "The diagram can't be displayed because it has too many nodes. Max allowed is 80."
+      when 413
+        "The diagram can't be displayed because it is too large."
       else
-        tag('img', src: URI('https://chart.googleapis.com/chart').tap { |uri|
-              uri.query = uriquery
-	    })
+        "Unknow error. Response code is #{response.status}."
       end
     end
   end