Other researchers have been investigating another potential link between global climate change and ragweed allergies. They hypothesize that global warming increases the overall length of the ragweed pollen season because warmer temperatures allow for a longer reproductive season for ragweed plants. They also hypothesize that this effect will be most pronounced at higher latitudes, where projections indicate that warming will be most severe.
To test this new hypothesis, researchers measured the length of the ragweed pollen season at various locations between Georgetown, Texas (latitude 30.63 degrees N) and Saskatoon, Saskatchewan, Canada (latitude 52.07 degrees N) in 1995 and again in 2009. They then graphed the change in the length of the ragweed pollen season (shown on the y-axis) against latitude (shown on the x-axis).
What relationship between these two variables would support the researchers' hypothesis?

First it is important to clarify that your question is incomplete, I do not see the graph you are talking about ..

Although faced with the situation, I took the trouble to investigate in medical pages on this subject, and I conclude that the hypothesis of the doctors is correct ...

Because greater global warming, lower partial pressures of oxygen in the atmosphere (therefore those people who had difficulty breathing will be more affected), greater pollutants in the environment (greater hypersensitivity reactions in allergy sufferers), and the seasons of Heat spread, therefore the pollination processes also and it is there where there is connection with allergies due to ragweed.

Explanation:

Allergies are increased immunological reactions against an antigen, so I added to your question that ambrosia pollen generates more allergies in the respiratory tract than at the cutaneous level, that is why it cooperates with other alterations that environmental contamination generates.

The greater the contamination, the greater the presence of allergies or the longer the allergies will be.