On the line graph settings screen, there are "Maximum Value" and "Minimum Value" boxes. Your Auto settings might be "ON". You can turn Auto settings "OFF" and specify values.
Let's say you are managing your weight. Your current weight is about 160lb and your target is 150. After a party or something, your weight goes up but has yet to exceed 165lb. On a day with no breakfast or lunch due to a heavy hang over, your weight may go down to 157lb in the evening.
In this case, I would set the Max and Min Value as 165lb and 145lb. With this setting, I get a line for the goal of 150lb, as well as the maximum limit of 165lb. You can see how close you are to your goal, or how close you are to your limit.
Give it a try. Turn off the auto settings, and you can have better views.
Good luck!