Biomedical research is highly data-driven. Domain experts need to learn how to interpret complex data visualizations to gain insights. They often need help interpreting data visualizations as they are not part of their training. Integrating visualization onboarding concepts into visual analytics (VA) tools can support users in interpreting, reading, and extracting information from visual presentations. In this paper, we present the design of the onboarding concept for an in- teractive VA tool to analyze large scaled biological data, particularly high-throughput screening (HTS) data. We evaluated our onboard- ing design by conducting a cognitive walkthrough and interviews with thinking aloud. We also collected data on domain experts’ visu- alization literacy. The results of the cognitive walkthrough showed that domain experts positively commented on the onboarding design and proposed adjusting smaller aspects. The interviews showed that domain experts are well-trained in interpreting basic visualiza- tions (e.g., scatterplot, bar chart, line chart). However, they need support correctly interpreting the data visualized in the scatterplot, as they are new to them. Another important insight was fitting the onboarding messages into the domain’s language