Add graph to constructor and store it

* Required for getting the number of colors used in coloring

src/SparseTransform/ConversionManager.cs

@@ -40,7 +40,7 @@ namespace SparseTransform
             }
             else
                 color.PartialD2Color((BipartiteGraph)graph);
-            ColoredDotWriter writer = new ColoredDotWriter(textColor);
+            ColoredDotWriter writer = new ColoredDotWriter(graph, textColor);
             return writer.Write(graph);
 
         }

src/SparseTransform/Convert/ColoredDotWriter.cs

@@ -7,6 +7,7 @@ namespace SparseTransform.Convert
     /// </summary>
     public class ColoredDotWriter : DotWriter
     {
+        private IGraph _coloredGraph;
         private bool textcolor;
         private string[] dictColor = new string[65];
        
@@ -14,8 +15,9 @@ namespace SparseTransform.Convert
         /// Constructor constructing a color-dictionary for coloring purpose.
         /// </summary>
         /// <param name="textColor">decision whether the labels should be colored or labelled.</param>
-        public ColoredDotWriter(bool textColor)
+        public ColoredDotWriter(IGraph graph, bool textColor)
         {
+            this._coloredGraph = graph;
             this.textcolor = textColor;
 
             for (int i = 0; i <= 3; i++)