Good question - I think a lot of people try too hard to impress the interviewer here and therefore make mistakes.
You are being tested for three things:
1) Can you read a chart?
2) Can you draw information that is relevant to the case
3) Can you use the information to suggest various ideas (depends on the question)
You'll notice that as you move from #1-3, you move from a more closed question to a more open-ended question.
Therefore - this is the key:
1) Start with the simple task of demonstrating you know how to read a chart by reading the title, the axes, and what are the main trends or pieces of information in the chart
2) Then draw out some information that you can discuss with the interviewer
3) Only then should you pause and try to come up with a structured set of creative ideas
If you go straight to #3, you are risking going down the wrong path and failing to demonstrate #1. Also, would you ever go straight to #3 in a real-life consulting engagement? Align with the team on #1 first! Just don't spend too much time on #1.
Finally, don't forget the question that was asked! Sometimes the interviewer will say, "The client brought you this data. What does this imply?" in which case it's much more open ended. But sometimes the interviewer will make it easier for you by asking, "What does this chart tell you about the proposed strategy?"
Helpful? Let me know what you think!