Genome-wide association studies (GWAS), which produce huge volumes of data, are now being carried out by many groups around the world, creating a need for user-friendly tools for data quality control (QC) and analysis. One critical aspect of GWAS QC is evaluating genotype cluster plots to verify sensible genotype calling in putatively associated single nucleotide polymorphisms (SNPs). Evoker is a tool for visualizing genotype cluster plots, and provides a solution to the computational and storage problems related to working with such large datasets.