Scientists have detected a small but non-negligible decline in global terrestrial “net primary production.” NPP is a way of measuring plant growth — how much carbon they’re removing from their surroundings and turning into biomass. Hence just by knowing what NPP is we can say that there exists a negative correlation between CO₂ and NPP. 

On first glance of the figure below (published in the article at http://www.sciencemag.org/content/329/5994/940.full) it looks like NPP and CO₂ levels are closely correlated.

9 out of 10 people will read the legend and understand what the scientists are trying to tell however there might be a few people who will look at the plot directly without reading the legend and they would feel that NPP and CO2 are positively correlated, which would be misrepresentation of facts.

Problem in the graph:

• The decision to invert the CO₂ scale.

• The graph is complicated to understand at the same time the colors are not suitable for color blind people.

Solution:

• A simple scatter plot would show the correlation. 

Graph Obtained by Plotting (CO₂ versus NPP) in Excel

The Equation of the line is NPP = -1.10054*(CO2) -0.0944566

We can clearly see from the scatter plot that there exits Negative Co-relation exists between NPP and CO₂