Allow me to re-phrase: "What is the crooked line at the bottom of the y-axis" I believe means "What is the purpose of a crooked line on the y-axis near the origin (0,0) ?"
It is used to indicate some omitted values on the y-axis. Foe example, suppose you are plotting the blood pressure of all of the patients for a given day at a doctor's office, If you are using the y-axis to show the numbers for the blood pressure (with time of patient visit on the x-axis, perhaps). then you do not need any numbers on the y-axis that are less than 30. (I would assume if the machine registers a "30" or less, than it is faulty or the patient is a corpse).
Therefore, you may utilize a squiggly line to indicate that some numbers are omitted, just to save some space on your vertical axis when you create the plot. It is never necessary to do this, just sometimes useful when no data exists in a certain range. This can also be done on the horizontal, or x-axis. Using the example above, suppose the doctor's office opens several hours later on Thursday, than Wednesday. You may then not wish to show time periods with no patients during 8:00 AM until 12 noon since the office opens at noon on that day.
Be careful to observe such "squiggly lines" on graphs. Otherwise you may misinterpret what you are viewing.