A very good observation and a matter of serious concern for presenters. Very often we take these things lightly and ‘tweak’ with our axis to highlight something which will never come to the fore if we start our axis from zero.

Should we always start our axis at zero?
I would say yes. It is always better not to distort the view of the audience to suit to one’s own liking.

However, if you do start with a non-zero axis you should inform your audience about it. I have seen umpteen charts, especially in annual reports and investor presentations, where axis starts with non-zero and the presenter never lets you know that the axis used is not starting at zero.

Look at Example 3b in the post below. I feel the change of axis is justified. However, what is missing is that the audience has not been intimated of the change.

http://www.allaboutpresentations.com/2009/09/14-charting-tips-tip-8-chart-axis.html

Regards,
Vivek

A very often used tool in “lying” with graphics is changing the vertical scale. When a complete graph increases from 12 to 14 over a 5 year period, this does not look dramatic. A much more dramatic effect is reached when the vertical axis starts at 11. This effect can be used both ways, depending on which message you want to convey.