For some time I worked for someone who disliked graph legends and preferred putting labels for lines and bar directly in graph. It is definitely easier to read in many cases, especially with line graphs.
After my ordeal with adding markers to a line, I wasn't optimistic about easily adding the labels where and how I wanted them in my Football Player Retention graph, but thankfully, I was pleasantly surprised.
First, I dragged the Grad Class dimension to the Label square of the Marks pane (in the All section). I then left-clicked the Label square to pull up a formatting pane. This allowed me to customize the text - I set to say "Grad Yr" in front of the number coming in from the data. I then moved down to Marks to Label and selected Line Ends. Additionally, I could specify if the labels were to be placed at the start and/or the end of the line. Awesome!