Focus on relative change, not on inventing absolute values the chart does not provide.

An index of 120 means the metric is 20% above the base period, while 80 means it is 20% below. That lets you compare direction and magnitude even when you do not know the underlying units.

Be clear about what you cannot conclude. You may know that one segment grew faster than another without knowing which segment is larger in absolute dollars.

Translate the relative movement into the business question, then ask for absolute data only if the decision truly requires it. Good analysis uses the information the exhibit supports without stretching beyond it.