下記のようなエラーが出ます。
このようにエラーが大量な文で出る時がありますがこれはなぜでしょうか？

この文は木を可視化するコードです

from sklearn.tree import export_graphviz

export_graphviz(tree, out_file="tree.dot", class_names=["malignant", "bennign"],feature_names=cancer.feature_names, impurity=False, filled=True)

import graphviz

with open("tree.dot") as f:
    dot_graph = f.read()
graphviz.Source(dot_graph)    

エラー

FileNotFoundError                         Traceback (most recent call last)
/Users/tadashintaro/anaconda/lib/python3.6/site-packages/graphviz/backend.py in pipe(engine, format, data, quiet)
    153             stdout=subprocess.PIPE, stderr=subprocess.PIPE,
--> 154             startupinfo=STARTUPINFO)
    155     except OSError as e:

/Users/tadashintaro/anaconda/lib/python3.6/subprocess.py in __init__(self, args, bufsize, executable, stdin, stdout, stderr, preexec_fn, close_fds, shell, cwd, env, universal_newlines, startupinfo, creationflags, restore_signals, start_new_session, pass_fds, encoding, errors)
    706                                 errread, errwrite,
--> 707                                 restore_signals, start_new_session)
    708         except:

/Users/tadashintaro/anaconda/lib/python3.6/subprocess.py in _execute_child(self, args, executable, preexec_fn, close_fds, pass_fds, cwd, env, startupinfo, creationflags, shell, p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite, restore_signals, start_new_session)
   1325                                 err_msg += ': ' + repr(orig_executable)
-> 1326                     raise child_exception_type(errno_num, err_msg)
   1327                 raise child_exception_type(err_msg)

FileNotFoundError: [Errno 2] No such file or directory: 'dot'

During handling of the above exception, another exception occurred:

ExecutableNotFound                        Traceback (most recent call last)
/Users/tadashintaro/anaconda/lib/python3.6/site-packages/IPython/core/formatters.py in __call__(self, obj)
    309             method = get_real_method(obj, self.print_method)
    310             if method is not None:
--> 311                 return method()
    312             return None
    313         else:

/Users/tadashintaro/anaconda/lib/python3.6/site-packages/graphviz/files.py in _repr_svg_(self)
     99 
    100     def _repr_svg_(self):
--> 101         return self.pipe(format='svg').decode(self._encoding)
    102 
    103     def pipe(self, format=None):

/Users/tadashintaro/anaconda/lib/python3.6/site-packages/graphviz/files.py in pipe(self, format)
    118         data = text_type(self.source).encode(self._encoding)
    119 
--> 120         outs = backend.pipe(self._engine, format, data)
    121 
    122         return outs

/Users/tadashintaro/anaconda/lib/python3.6/site-packages/graphviz/backend.py in pipe(engine, format, data, quiet)
    155     except OSError as e:
    156         if e.errno == errno.ENOENT:
--> 157             raise ExecutableNotFound(args)
    158         else:  # pragma: no cover
    159             raise

ExecutableNotFound: failed to execute ['dot', '-Tsvg'], make sure the Graphviz executables are on your systems' PATH

Out[177]:
<graphviz.files.Source at 0x12958e400>

Macを使っています。Python のバージョンは 3.6.1 で、Anaconda を使ってインストールしました。

追記

ターミナルで dot -V を打った場合 command not found でした。