Does this graph mean what it is supposed to mean for the joke to work? The black line means I learn much quicker with less time investment, i.e. it is easier than all the others.
That is a common misunderstanding of how learning curves work. A steep learning curve means your skill increases more rapidly with the invested time. That means the subject is easier or more intuitive.
You nearly had it. The black line starts higher at "gaining skill" so it requires more skill to start learning but after short time you are gaining much more skill in the same time.