If the solution is isotonic to the sap cells in the plant, there should be no difference before and after the experiment. This means that the movements of the substance from and into the sap cells are even and stable. Therefore, there is no observable change in length/size, it remains constant.
So, according to the graph (I know how it looks), the concentration of glucose solution that is isotonic to the sap cell, is on axis-x, which means 0 percentage change in length of the plant sample.
•If it is hypotonic, the points drawn on the graph should be above axis-x, which means there is a positive change
•If it is isotonic, the points should be on axis-x which means there is no change
•If it is hypertonic, the points should be under axis-x, which means negative change