Add anchor nodes

Nodes set as anchors won't be affected by the force directed layout.
They can be used to fixate the graph to a certain area.

fd/Graph.lua

@@ -148,12 +148,14 @@ function Graph.new()
                 nodeA:attractTo( attractionPoint );
             end
 
-            for _, nodeB in pairs( nodes ) do
+            if not nodeA:isAnchor() then
-                if nodeA ~= nodeB then
+                for _, nodeB in pairs( nodes ) do
-                    nodeA:repelFrom( nodeB );
+                    if nodeA ~= nodeB then
+                        nodeA:repelFrom( nodeB );
+                    end
                 end
+                nodeA:move( dt );
             end
-            nodeA:move( dt );
 
             minX, maxX, minY, maxY = updateBoundaries( minX, maxX, minY, maxY, nodeA:getPosition() );
         end
@@ -213,6 +215,11 @@ function Graph.new()
         return ( ( maxX - minX ) * 0.5 ) + minX, ( ( maxY - minY ) * 0.5 ) + minY;
     end
 
+    function self:setAnchor( id, x, y )
+        nodes[id]:setPosition( x, y );
+        nodes[id]:setAnchor( true );
+    end
+
     return self;
 end
 

fd/Node.lua

@@ -16,6 +16,7 @@ function Node.new( id, name, x, y )
     local self = {};
 
     local name = name or id;
+    local anchor = false;
     local px, py = x or 0, y or 0;
     local ax, ay = 0, 0;
     local vx, vy = 0, 0;
@@ -100,6 +101,14 @@ function Node.new( id, name, x, y )
         px, py = nx, ny;
     end
 
+    function self:setAnchor( nanchor )
+        anchor = nanchor;
+    end
+
+    function self:isAnchor()
+        return anchor;
+    end
+
     return self;
 end