For continuous responses, the Actual by Predicted plot is the typical plot of the actual response versus the predicted response. When you fit a Decision Tree, all observations in a leaf have the same predicted value. If there are n leaves, then the Actual by Predicted plot shows at most n distinct predicted values. The actual values form a scatter of points around each leaf mean on n vertical lines.

The diagonal line is the Y = X line. For a perfect fit, all the points would be on this diagonal. When validation is used, plots are shown for both the training and the validation sets. See Figure 4.15.