Under an evolution graph, the "Rows to display" value is not saved and restored on subsequent view of this evolution graph.
This feature is very useful and the selected value is expected to be remembered & persisted on subsequent views of this particular data set.
We already have implemented this for other visualisations where we store the preference for example in visits log and table reports.
Note: The value should be recorded for the particular evolution graph and affect only this graph (for example Visits Overview graph can be set to a different value than Referrer Overview graph). In the case of Goal evolution graphs, the same "Rows to display" value should be used on all Goal Evolution graphs (independantly of idgoal).