For an early study of the relationship between diet and heart disease, the investigator obtained data on

Question:

For an early study of the relationship between diet and heart disease, the investigator obtained data on heart disease mortality in various countries and on national average dietary compositions in the same countries. The accompanying graph shows, for six countries, the 1948-1949 death rate from degenerative heart disease (among men ages 55-59 years) plotted against the amount of fat in the diet.
In what ways might this graph be misleading? Which extraneous variables might be relevant here? Discuss.