I suppose that you are referring to Send an email
chapters in the documentation that seem incomplete to you. And you are right, passing the graph parameters is not literally mentioned there and it is definitely a good idea to improve that.
If you want to display a graph parameter in an email sent as a response to a job execution, you can simply add a line to the HTML email definition:
Where PARAM1 is the name of the parameter as it is defined in the "Edit graph parameters" screen in the Designer. In the resulting email, you will then see whatever is the value od the PARAM1 parameter. In my example graph it is e.g.:
AnyName: Hello World!
Please let me know in case I have misunderstood your question.