I got this "parabolic" curve from a book but cannot find the right equation for it The diagram below is taken from a book on Indian Stupa architecture. It says that the profile is a "parabolic" one. I have tried y=x^2 and varied the domains ox x and y but couldn't find the right proportions. I actually don't think it is a parabolic curve. Which other function could it be? Is the grid behind of any help?

I have tried a logarithmic version of the grid but it doesn't quite fit:

This one is with the following forumla suggested by Hypergeometric:

$$\Large x=r\left(1-2^{\frac y2}\right)$$


Here is an attempt with the following equation suggested by Hypergeometric combined with an evolutionary solver to find the closest variables and it is getting much closer (domain used r(1,5.598), m(1,0.980), y(1,5.608)) : 

$$\Large x=r\left(1-m^{\frac yn}\right)$$



It was parabolic but I should have put the exponent as a variable on y apply a scaling on the x axis i then used an evolutionary solver to find both scaling factor and exponent:

$$\Large x = t a$$

With scaling factor a = 0.17369

$$\Large y = t^b$$

With exponent b = 0.2787
The domain of t is 0 to 1

