When performing a large number of Yield Analysis iterations, some traces on a graph will disappear after the Yield Analysis simulation is done. In other words, there are more Yield Analysis traces on a graph during the simulation than after the simulation has stopped.
Each trace on a graph has a large number of points. In the presence of many traces on a graph, as is the case during Yield Analysis with many iterations, the software conserves memory by displaying fewer of these traces (each trace corresponding to a Yield Analysis iteration).
The overall answers of the yield analysis are not affected, and the user can view the min/max of the traces by right clicking on the graph and selecting Properties navigating to the Yield Data tab and enabling the Show Range box or the Show Sigma boxes.