Price Elasticity Curve

In RDF, when we are computing the price elasticity, it takes 3 inputs for coming up with the variation namely the Price Elasticity, Decay Factor and the Effective Period.

These inputs give us a variation which is shown in the user guide as given below. The shape of graph is governed by the decay factor.

Here it is seen that the volume increases when there is a price decrease and then it goes back to the baseline level. The decrease is more initially and becomes less towards the end of effective period.

Is there any specific why we have this shape of graph? Is it possible to get a curve which has slow decrease initially and becomes more towards the end? ( A more convex curve than a concave curve)