Placement of nodes in a nested subgraph in Graphviz

I am trying to create a plot of a binary, coloring and marking different nodes in different colors.

To get the borders around the subtrees I use subgraphs which works almost perfectly fine:

graph G
{
graph [ranksep="0.25", nodesep="0.25"]
rankdir = TB;
node [shape=ellipse, style=filled, fillcolor="#0068B4", color=white, fontcolor=white, penwidth=10]
edge [arrowtail="none"]

subgraph cluster_0 {
  node [fillcolor="#99CC00"]
  color="#99CC00"
  style=filled
  fillcolor=white
  fontcolor="#99CC00"
  label="12 is the root of\n26's left child\r"
  18;
    subgraph cluster_01 {
      node [fillcolor="#00B0F0"]
      color="#00B0F0"
      fontcolor="#00B0F0"
      style=filled
      fillcolor=white
      label="4 is the root of                         \n12's left child                         \r"
      7   [fillcolor=white]
      4 -- 13
      4 --  7 [style=invisible]
      7 -- 13 [style=invisible] 
      { rank=same; 7, 13 }
      { rank=same; 4, 18 }
      
    }


  12 -- {4,18}
  
  { rank=same; 12 }
}

subgraph cluster_1 {
  node [fillcolor="#C00000"]
  color="#C00000"
  style=filled
  fillcolor=white
  fontcolor="#C00000"
  label="32 is the root of\n26's right child\r"
  
  35   [fillcolor=white]

  32 -- 38
  32 -- 35 [style=invisible]
  35 -- 38 [style=invisible] 
  
  { rank=same; 32 }
  { rank=same; 35, 38 }
}


26 -- {12, 32}


{ rank=same; 26 }

}

which leads to this output:

As you can see, the node 18 is placed inside the blue subcluster. Is there an easy way to move that node "out of the box" other than placing a blank dummy node (like the invisible nodes 7 and 35) between 4 and 18?

Solution

But, adding graph [newrank=true] and rearranging a few lines makes things better:

graph G
{
graph [ranksep="0.25", nodesep="0.25" newrank=true]
rankdir = TB;
node [shape=ellipse, style=filled, fillcolor="#0068B4", color=white, fontcolor=white, penwidth=10]
edge [arrowtail="none"]

subgraph cluster_0 {
  node [fillcolor="#99CC00"]
  color="#99CC00"
  style=filled
  fillcolor=white
  fontcolor="#99CC00"
  label="12 is the root of\n26's left child\r"
    subgraph cluster_01 {
      node [fillcolor="#00B0F0"]
      color="#00B0F0"
      fontcolor="#00B0F0"
      style=filled
      fillcolor=white
      label="4 is the root of                         \n12's left child                         \r"
      7   [fillcolor=white]
      4 -- 13
      4 --  7 [style=invisible]
      7 -- 13 [style=invisible] 
      { rank=same; 7, 13 }
    }
      { rank=same; 4, 18 }
  12 -- {4,18}
  { rank=same; 12 }
}

subgraph cluster_1 {
  node [fillcolor="#C00000"]
  color="#C00000"
  style=filled
  fillcolor=white
  fontcolor="#C00000"
  label="32 is the root of\n26's right child\r"
  35   [fillcolor=white]
  32 -- 38
  32 -- 35 [style=invisible]
  35 -- 38 [style=invisible] 
  { rank=same; 32 }
  { rank=same; 35, 38 }
}
26 -- {12, 32}
{ rank=same; 26 }
}

Gives: