Starting the value axis of a chart – usually the y-axis – at a non-zero value can visually exaggerate the initial insights that the reader draws. In example 1 below, the first instinct is to think that there is a >80% difference between 2010 and 2014. Once we extend the y-axis to 0, however, we see that the differences are quite minor (example 2).
That’s not to say that all charts should have a value axis that starts at 0. If, for example, we were looking at temperatures in the charts above starting at 0 Kelvin would make little sense. In general, the value axis range should include the reasonable range of the data and not deceive the reader. If, for example, we were looking sales data above it is reasonably plausible to have 0 sales and therefore the y-axis should likely extend to 0.
On the other hand, in a certain scenario a business analyst may have found out that a large share of sales is fixed. In that case she may want to focus on the fluctuations above a certain level like in example 1 above. So the right way of presenting the data always depends on the story it is supposed to tell and the insights the designer wants to share.
Advanced chart tools like Aploris have the ability to edit the format of an axis to ensure it appropriately depicts the chart data and supports the message to the reader. This includes editing the axis range and tick mark intervals, using 100% and logarithmic scales as well as inserting axis breaks.