Why? I think there's some intuitive idea that something is being hidden or there's cheating in the presentation, but that doesn't have to be the case if the purpose of the plot is to illustrate relative change (and the y axis is clearly labeled).
The counter-argument is that it makes it look like CO2 has 10x'd instead of doubling, but since those values aren't calibrated in units that we can intuitively understand, it's not obviously the correct choice to start at 0.
I understand that for line graphs, where you are looking at variations rather than absolutes, this is acceptable, but in this case.
- This is an interactive graph, at least give us the option of starting at zero.
- CO2 went from 280 ppm to 420 ppm, it is significant enough to see the variations even on an absolute scale.
- Don't make the bottom line (x axis) stand out as it does now, especially considering that the y axis doesn't stand out.