You need to choose a scale that is meaningful for your users. You say that a person who has a healthy diet will always have above 80.
Does it make a significant difference to have 82 or 95? Significant means that the user will need to take different actions. If not, then your scale is far too precise for user's needs. By focusing the chart on the top of the scale, you reinforce the idea that minor changes (between 86 and 94) are meaningful, when they might not be.
Worse, your users might get some motivation to see their scores always at the top of the chart. By zooming in on the top of the scale, they lose the perspective of their overall healthy eating habits, to focus on day-to-day variations.
In the end, it depends on what you want your users to focus on.