[raquo]

Because when zero is not marked it is impossible to judge the relative magnitude of all graph fluctuations without looking at the y axis scale. On this graph a lot of people (incl. me) would be tricked into thinking that in the last 100 years the amount of CO2 increased by an order of magnitude. Now, I know that's impossible, and I checked the scale at the left to find the usual suspect. But many normal people would just believe what they see. It's typical deceit method that is widely used in corporate presentations, journalism, etc. The only excuse for not marking zero on most charts is if you want to show minor fluctuations more precisely, which is not the purpose of this graph.

[lutorm]

The magnitude of the fluctuations should really be judged against the error bars, not against zero. That's what tells you whether the change is significant.

[sesqu]

But that doesn't tell you whether it's important. Statistical significance is a measure of trustworthiness, given assumptions about the generating process. It tells you nothing about whether the data is in any way meaningful, after assuming relibility.

What does tell you about that type significance of (rational) data is the delta, which requires the 0 in order for you to see the scale. Now, in many interesting cases this could be represented as error bars under H0, but that's usually not done in publications, because they only have room for one plot type.

The range of the plot is not arbitrary. To suggest otherwise is deceptive.

[lutorm]

But that doesn't tell you whether it's important

Yeah, I'll buy that.

[natnat]

When you're looking at a line chart that starts at zero, it's very difficult to really understand what's going on unless there's a really huge amount of variability in the data. For example, charts of stock prices virtually never start at zero. Does that mean that, say, Google Finance is trying to deceive you into thinking that Apple's price went up ten times today? No, it means that they're assuming that you're not stupid and that you know how to read a chart. NASA very clearly labels the chart, and anyone who isn't astoundingly stupid or willfully trying to misunderstand the data can see very clearly that this chart starts at 160.