Python Decision Tree code not producing Image

[AENB2016]

Hello,

I'm trying the Python Decision tree example code from this website:

https://medium.com/@rnbrown/creating-and-visualizing-decision-trees-with-python-f8e8fa394176

All lines execute without any problems except the very last one to produce the image. When I run it I get the following error message:

InvocationException: GraphViz's executables not found

I'm pasting the full code below. 

Thanks

import sklearn.datasets as datasets

import pandas as pd

iris=datasets.load_iris()

df=pd.DataFrame(iris.data, columns=iris.feature_names)

y=iris.target

from sklearn.tree import DecisionTreeClassifier

dtree=DecisionTreeClassifier()

dtree.fit(df,y)

from sklearn.externals.six import StringIO  

from IPython.display import Image  

from sklearn.tree import export_graphviz

import pydotplus

dot_data = StringIO()

export_graphviz(dtree, out_file=dot_data,  

                filled=True, rounded=True,

                special_characters=True)

graph = pydotplus.graph_from_dot_data(dot_data.getvalue())  

Image(graph.create_png())