A visualization of nightly regression test results provides rapid feedback, and may aid in making decisions on software projects. However, visualizing test results from sparse and distributed nightly regression testing is non-trivial. We have conducted a structured literature review focusing on how heatmaps are best used for software test data visualization. By identifying existing studies, we have discovered how stakeholders use the visualizations, how they are valuable, the approaches used for plotting and the proposed future research. The review identified four papers that describe visualizations that target managers, experts, testers and code maintainers. The visualizations are used for making decisions, giving support, or provide early warnings. Typically the source code of the software under test is an important part of the plots.