Analyzing executions of concurrent software is very difficult. Even if a trace is available, such traces are very hard to read and interpret. A textual trace contains a lot of data, most of which is not relevant to the issue at hand. Past visualization attempts either do not show concurrent behavior, or result in a view that is overwhelming for the user. We provide a visual analytics tool, VA4JVM, for error traces produced by either the Java Virtual Machine, or by Java Pathfinder. Its key features are a layout that spatially associates events with threads, a zoom function, and the ability to filter event data in various ways. We show in examples how filtering and zooming in can highlight a problem without having to read lengthy textual data.