In order to apply a different stroke to part of a series, you'll need to split the line up. First, select the line and its points and change the connection type to "none" using the style inspector - you'll be left with unconnected points.
Select just the points of the graph which you want to be connected with the special stroke, and turn the connecting line back on using the inspector. Set the special stroke.
Then select the remaining points and connect them in the same way, choosing the a different stroke.
Select just the points of the graph which you want to be connected with the special stroke, and turn the connecting line back on using the inspector. Set the special stroke.
Then select the remaining points and connect them in the same way, choosing the a different stroke.