A little late for this question, but thought I give it a shot, since I use charts in my cases often.

What's the biggest mistake I see? Looking for patterns in values. To give an example: Let's say, I show a simple bar chart for an white goods company, depicting fluctuating sales numbers (in units) for refrigerators and washing machines from 2000-2015. Very often, candidates come up with answers like this: "One can see that sales overall were very flat over time, average probably 300,000 units a year, except for the big drop in 2009, maybe because of the financial crisis. Interestingly the share of washing machines is quite constant at 60% of units over time".

Why is this bad?

• No insight - it's from the stating the obvious department. The candidate explains to people who are responsible for these numbers as their main job HOW these exact numbers LOOK to an outsider. Believe me, they know.
• Wild accusation (financial crisis). (don't confuse with business judgment!) [This always amuses me, I just update my case charts by 1 year each year, and candidates try to connect the data to historic events]
• It's out of context, no idea why the candidate is doing what he's doing or where he's heading (assuming the chart is part of a case and not the exercise itself)

It's obvious that candidates have no clue what to do, but this is the worst way to buy time. OK, since you asked, how SHOULD you do it?

Key elements of any chart are: Title, axes descriptions (units!!), and legend. The values are far, far less important, in the end they only go into the equations. My favorite approach:

1. Say title, axes descriptions with units, and legend out loud. Medium to slow tempo to buy thinking time.
2. Connect it to whatever the hell you were doing BEFORE you got totally confused by the chart. If necessary, repeat what you are aiming for at this point in the case structure.
3. Pick the elements/numbers from the chart you actually really need for 2. Chances are, a large chunk of the data is completely useless. Making sense out of a mess is a core consulting skill and tested for in interviews.
4. Continue with the case to the next point.

How would a good answer look like? "On this chart, I see the sales number of refrigerators and washing machines from 2000-2015 for the client in thousand units. In order to now estimate the sales potential for the new detergent line as add on, I need to estimate the number of washing machines of our client's brand in households. I would thus take the sales numbers from ..."

--> Repetition buys time and brings clarity (= shows your understanding to the interviewer, and thus your listening skills), and is not "wrong". Repeating the "why" shows you're in control and managing the situation, heading to solve the case.

This is a trick consultants use all the time: Repeat facts or repeat the overall goal. :) For steps 1 and 2, you don't yet need to think AT ALL, bring any prior knowledge, or even understand the chart. And you still sound smart and have a bit more time to figure things out.

Hope this helps, good luck for the interviews!