Here are some pieces of advice on how you can interpret graphs and charts.
To interpret a graph or chart, read the title, look at the key, read the labels. Then study the graph to understand what it shows.
Read the title of the graph or chart. The title tells what information is being displayed.
Look at the key, which typically is in a box next to the graph or chart. It will explain the symbols and colors used in the graph or chart
Read the labels of the graph or chart. The labels tell you what variables or parameters are being displayed.
Draw conclusions based on the data. You can reach conclusions faster with graphs than you can use a data table or a written description of the data.
To practice all these bullets GMAT Integrated Reasoning questions help you a lot!