This diagram every value is represented with respect to a central point. All the changes in the values are expressed in the form of distance from this center point.
The radar diagram given below represents the highest value of two indices over a given period.
It can be seen that the centre value = 8000. With every passing circle the value increases by 2000. Since here are seven years, the diagram takes the shape of a heptagon.
Had there been only six years, it would have taken the shape of a hexagon, as shown below:
Similarly, had there been only four years, the diagram would have been in the shape of a square.