One important thing to note about that graph is that it is a graph of bachelor's degrees and not a graph of careers. Some people have suggested the turning point in the graph might actually reflect a reclassification of "computer science," saying that before 1984, data entry clerks and secretaries doing word processing were counted under "computer science," and the downward curve just represents a redefinition of the word to include only actual programmers and such.
But it's a graph of bachelor's degrees, and you don't get a bachelor's degree to do word processing or operate a keypunch. I can tell you this because my mom was a keypunch operator. She did have a bachelor's degree, but it was in diplomacy, and she was finishing up her master's in special education. These are generally not considered computer science degrees.
In some of the worst life advice anyone has ever given, her sister encouraged her to pursue a career in special ed because it would be an easy way to make money. My mom, like most people, was not cut out for teaching special needs children. On her first day as a student teacher, as soon as her mentor left the room, all the students fled out the windows, leaving her at a complete loss. (She didn't specify when she first told me, but these were first-story windows, making it a less-interesting but also less-tragic story.)
At the same time, she had a part-time job as a keypunch operator, which is the exact same mind-numbing job that data entry is today -- read a form, type the words onto a machine. The only difference is the typing made holes in a card that you put into a giant computer, which is very funny today and gives us a good laugh at how backwards people were in the '70s.
Punch cards are the computer version of leisure suits.