This quote can be found only in the movie.
It can be heard in the third movie, and is told by the headmaster.
I just checked the book, and it doesn't appear there.
I think it would be a because you don't use diagrams to show graphs because they are themselves a diagram
There are many factors that could hamper an effective communication. Some of those could be: social status (if you are talking to someone who has a more important status than yourself, it may happen that they are not interested in a communication with you), use of language (if you use inappropriate words, the conversation is not going to be successful), knowledge about the subject (if you don't know what you are talking about, the conversation is likely not going to continue), atmosphere/noise, cultural differences, disabilities, etc.
To be non-biased, have thorough research habits, a good work ethic, to produce the story as closely to the true events as possible (no filler or otherwise distracting language to take attention away from the topic)
