> You can form an opinion about the density of these websites simply by looking at an image for a franction of a second.
Those little + make the left graph much more expert user friendly. A chemist isn't looking at the graph to see that there are peaks (they know that, they're chemists), they want to find actual points on the line and the + are enabling a level of eyeballing on the low-density graph that isn't possible on the right.
If I'm a heavy graph user, I want the low density one. The high density one isn't for daily use - it is for appreciating as a one-off or learning about periodicity of elements.
Tufte's minimalist graph is much better if it is meant to tell a story or be shown to an unsophisticated audience. But if someone wants to actually refer to the data on a regular basis some guides are a much better approach that will cut down on stupid mistakes. Eyes aren't very good at scanning over blank space without drifting.
Isn't it the other way around? High on the left and low on the right?
EDIT: Based on the alt text both images should be swapped.