A data display firmware improvement for a scan tool adds the capability to view in a single presentation data uploaded from a motor vehicle's onboard diagnostic (OBD) computer synchronized with data from external test apparatus. The data display algorithms permit viewing of OBD data, including certain real-time oscilloscope-like waveforms, and allow viewing test results from compatible instruments, such as exhaust gas analyzers, including capturing realtime OBD signals, holding them until delayed events such as gas analyzer test results become available, then displaying the events on a common display as though available simultaneously in real time. The firmware upgrade is downloadable to existing products.