The One Factor Effects graph shows the linear effect of changing the level of a single factor. It is constructed by predicting the responses for the low (-1) and high (+1) levels of a factor.
For a more general set of graph tools see Model Graphs - General Information
For factorial analysis, the I-beams around the predictions are the result of least significant difference (LSD) calculations. The LSD I-beams are approximate tests to check if the predictions at the displayed factor combinations are significantly different. Click on a prediction symbol to draw the graphs with individual LSD bars for pairwise comparisons. Click any where else on the graph to bring back the I-beams.
By default Design-Expert will display a warning on one factor plots for factors involved in interactions. When there is an interaction, the factor’s effect depends upon the settings of other factors. One Factor plots can be very misleading when there are interactions.