Andrew, the graph indicates mortality rate. If they die at an increased percentage the alleles they carry die with them. Thus you have allele frequency change in the population.
In this case the Hybrids are preserved over the AA dominant type and the S allele increases. Whereas, in a non-malarial area, the A allele increases in frequency. In both cases evolution occurs and the populations in non-malarial areas diverge from the malarial areas.
Then why is the Hybrid frequency only 15-25% in a malarial area?
THE GEOGRAPHY OF SICKLE CELL DISEASE:
West Africa
The sickle trait frequency varies from 15% to 25% and the HbC trait frequency from 3% to 4% in Nigeria to 15% to 20% in parts of Ghana and Burkina Faso, so the predominant genotypes of sickle cell disease are SS and SC disease, the latter being proportionately more frequent in Ghana. Data on Sb+ thalassemia and Sbo thalassemia are insufficient to predict their relative prevalences. Alpha+ thalassemia is common. Malaria is common and general public health measures poor outside major cities. High socioeconomic status improves survival,55 ensuring easier access to medical care, better public health measures, better immunization and nutrition, and better malarial prophylaxis. Few figures are available but mortality is high and survival to adult life uncommon. Most mortality is attributable to malaria or other infections. Leg ulceration and other end organ damage such as renal failure are uncommon, presumably because patients do not survive long enough to develop these complications. Pregnancy is also infrequent and pregnancy related pathology consequently rare in SS disease although more common in SC disease.