Design Principles
A visualization must be easily understandable by everyone who sees it. This article demonstrates some ways to ensure that charts are understandable and engaging.
Do
Do use the full axis
Avoid distortion.
For bar charts, the numerical axis (often the y axis) must start at zero. Our eyes are very sensitive to the area of bars, and we draw inaccurate conclusions when those bars are truncated. See the difference between the original media chart and an un-truncated chart as generated by FlowingData.
(But for line graphs, it may be okay to truncate the y axis.
Wide ranges:
If you have one or two very tall bars, you might consider using multiple charts to show both the full scale and a "zoomed in" view - also called a Panel Chart.
Consistent intervals:
Finally, using the full axis also means that you should not skip values when you have numerical data. See the charts below that have an axis with dates. The trend is distorted if you do not have even intervals between your dates. Make sure your spreadsheet has a data point for every date at a consistent interval, even if that data point is zero.
