Here is what a first time viewer needs to go through when looking at a data chart:

• What's the unit on the y-axis
• What's on the x-axis
• What do the legend colours stand for
• What is "good" or "bad", are things supposed to go up or down?
• What is actually happening
• Is the y-axis broken or does it start on zero?
• Why is this unexpected, interesting, counter-intuitive, amazing, terrifying?

If you did the analysis then you live and breathe the data in the chart, the only thing you watch out for is how the line/column/bar looks different from the previous version of the chart you saw. For a completely new audience it is a different story.

There are 2 types of data charts:

1. Charts to ponder and refine research and analysis
2. Charts to communicate the result of the pondering and refining

If you make a presentation of result, you deal with scenario 2. Make a diagram completely from scratch. Start with "what is actually happening" and only put that piece of data on the chart.

For example if a complex stacked line diagram shows that the % of returning visitors gets smaller over the past 5 years, you can replace all that monthly visitor data with 3 data points: % of return visitors in the last 3 years.

Image by Phil Roeder on Flickr