Visualizations of mass shooting incidents in the United States appearing in the media can influence people's beliefs and attitudes. However, different data sources each use their own definition of mass shootings, resulting in varying counts and trends of these incidents across the sources. To investigate the effects of these varying definitions on public perceptions, we conducted a crowdsourced study using data from four sources-Mother Jones, Mass Shooter Database, Everytown for Gun Safety, and The Washington Post. We used one or more line plots, with or without explicitly providing the definition, to see how these variations affect viewers' understanding of a 10-year trend in mass shooting frequency. We found that, depending on the data shown, participants' perceptions of the trend changed in both directions (i.e., more or less increasing) compared to their prestudy perceptions. We discuss how data from a single source can influence people's perceptions, and how visualizing data from multiple sources (e.g., superimposed line graphs) can enable more transparent communication. Our work has implications for other media and public visualizations, highlighting the importance of embracing pluralistic approaches to enquiry, especially when dealing with data of significant importance and consequence.