The investigators were attempting to control the affects of these potentially confounding variables. Patterns of sexual activity, e.g. number of partners, vary considerably by age, education, and marital status. The conclusions drawn by the investigators were not justified. Because sexual activity and number of partners increase the risk of cervical cancer, one would expect women using the pill to have a higher rate of cervical cancer. There is no reason to conclude that the pill was causing cervical cancer.
Histograms convey information about percentages by area.
This is remarkably even. The graph is a bar chart, not a histogram. There is no horizontal axis, just categories of various income ranges. The vertical axis is percentage, not percentage per income unit. Histograms convey information about percentages by area, not by height.