# How to display zero in Line Chart ?

What's better way to display zero value in a Line chart ? The A chart display zero value separate from X and Y axis.

I don't know the reason why because normally I see a lot of line chart like B :

What's is better ?

• Your comment on Chart A is not clear, can you please provide better picture for it? – Chetan Jul 13 '15 at 10:44
• Sorrry. I capture it from this page. c3js.org/samples/simple_multiple.html . You can see zero value is separate from X axis and Y axis. – Chung Jul 13 '15 at 14:35

The 0-value x/y axis (chart B) is such a standard charting convention that it's often presumed. Users just glancing at chart A would likely assume that any point above the x axis represents a positive value, which in this case would be an incorrect reading. While chart A is slightly cleaner aesthetically (no overlapping lines), I think the risk of confusing the user or misrepresenting data outweighs the potential value.

Under most circumstances, if you're dealing with a linear scale, always keep the zero mark directly on the axis.

Reason: The main purpose of a multi-line graph is to allow the reader to view and compare the values of different items across a variable, typically time. When 0 is on the axis, the reader can eyeball how far different items are from the axis to tell relative differences.

e.g. In Example B, it seems like the green trends at ~40% mark between blue and red with the exception of the huge spike at 4:34am.

When the 0 value is move above the axis, this is now a lot difficult to do... in fact, unless the reader is explicitly told the zero mark is above the axis, it's very easy to misread the graph.

If you feel values of an item is very close to the zero line and it'll be difficult see the small changes, consider using a different scale.

e.g. For roundtrip time, do you care as much about the exact values or fluctuations throughout the day? If it's fluctuations, consider figuring out a baseline value for each environment (perhaps the desired response time for each environment?), then graph % deviation from baseline.