Each circle in that chart is a county in the United States, with bigger circles representing bigger counties. The fitted lines tell us that as the proportion of people receiving mammograms goes up, the rate of people being diagnosed with cancer goes up, pretty dramatically. But the rate of people dying from breast cancer in 10 years is pretty much unaffected.
When analyzed at the county level, the clearest result of mammography screening is the diagnosis of additional small cancers. Furthermore, there is no concomitant decline in the detection of larger cancers, which might explain the absence of any significant difference in the overall rate of death from the disease. Together, these findings suggest widespread overdiagnosis.