Hans Rosling’s Ted Talk presentation serves as the classic example of how to clearly explain a data graphic to an audience. Watch this video:
He explains the axes, the colors, and the encodings. Listen for when he describes China as “the big red bubble” as a cue to point the audience’s attention to the country of interest.
Even if a legend is present, take the time to describe how the data is encoded. For example, what do the lines, points, filled areas, or bubbles mean? Label the units used, whether it be U.S dollars or Japanese Yen, hours or minutes, kilometers or miles, degrees Fahrenheit or Celsius, for example.
Take the time to explain the chart elements to your audience.