Hi,

You can use the following approach:

1. Take a minute to look at the graph
2. Read the graph title
3. Look at the graph type and define the type (pie chart, line chart, etc)
4. Look at the legend (ask for clarifying questions if necessary)
5. Identify whats going on on the graph. Look for: Trends, % structures,
6. Look for unusual things - correlations, outliers,
7. Make 3-4 conclusions from the graph. Think of potential hypothesis on what could be the root cause / what are the consequences
8. Prioritize the most important for your current analysis and move forward with the case

Sources to learn from (prioritized):

1. Study "Say it with Charts" book by Barbara Minto
2. Learn basic statistics (Any GMAT or MBA prep guides)
3. Check all available MBB presentations and publications. Practice to derive conclusions and check yourself with the actual ones from the article / presentation
4. GMAT IR part (Official guide and Manhattan prep)
5. "Consulting Bible" and "Vault guide for consulting" - check the chapters on cases with graphs in these books

Good luck!