title: Writing create_tree_digraph plot to a png file in Python
body: <p>I want the tree of my lightgbm model to save to a .png format. I have tried two plotting methods from lightgbm API - plot_tree and create_tree_diagraph.</p>
  
  <pre><code>import lightgbm as lgb
  from sklearn.datasets import load_iris
  
  X, y = load_iris(True)
  clf = lgb.LGBMClassifier()
  clf.fit(X, y)
  </code></pre>
  
  <p>When I use plot_tree, it displays the tree but in place of values there are small blank boxes</p>
  
  <pre><code>lgb.plot_tree(clf, tree_index=0)
  </code></pre>
  
  <p>When I try the create_tree_diagraph, I get the graph but I cant save it as it is. </p>
  
  <pre><code>lgb.create_tree_digraph(clf)
  </code></pre>
  
  <p>I used the below code to save it a file but that gets saved as the first plot (using plot_tree)</p>
  
  <pre><code>import graphviz
  s = graphviz.Source(graph_b.source, filename = "test1.gv", format = "png")
  s.view()
  </code></pre>
  
  <p>Any suggestions to save the plot as an image. I ultimately want to write these tree plots to excel.
  I am using graphviz version 0.8.3
  Thanks,</p>