Visual information can be very convincing. It is important that your visual information is fair. Here are some ways that data visualization may go wrong:
Vertical axis starts above zero - This creates a chart that magnifies the visual difference between two numbers, making slight increases or decreases look significant.
Missing figures - If there is no context for the visual information with numbers, there is no credibility.
Height versus width - Beware of a greater change in width versus height that can make the difference seem greater than it really is.
Changing the unit of comparison - This is comparing apples to oranges instead of apples to apples. Of course, if the chart doesn't label the unit of comparison, you wouldn't know if this is the case, which should be a red flag.
Changing the height of vertical values - This is when the height of a unit (like a bar on a chart) doesn't match up with the vertical axis.
Axis units are not consistent - Units should remain a consistent size or there is no visual frame of reference for increases or decreases.
Changing the conventions - People make assumptions about the information based on the type of chart used. Changing the conventional use can create a situation in which people are easily tricked.