What condition makes a Duane plot appear as a straight line?

A Duane plot represents reliability and failure data, typically used in the context of assessing the reliability of systems and components over time. The characteristic that makes a Duane plot appear as a straight line is the establishment of a constant failure rate, which correlates directly with the underlying assumption of the failure behavior of the system.

When the failure rate is constant, it indicates that failures happen at a steady rate over time, which aligns with the characteristics of an exponential distribution. In this context, the cumulative failure rate plotted against cumulative test time will yield a straight line, reflecting the linear increase of failures as time progresses under a constant failure rate condition.

The other options do not result in a straight-line plot on a Duane plot. While plotting cumulative failure rate against cumulative test time is essential in representing the data, it is the condition of the constant failure rate that ensures the linearity of the plot. Using a logarithmic scale modifies how data is presented, and graphing individual failure events does not form a continuous line necessary for the analysis typically done with a Duane plot.