There are many functions which follow some shapes like those. One idea would be 100*(x/100)^c with a free parameter c. c=1 gives a straight line, c=1/2 the square root.

If the derivative should vanish at x=100, you can try the circle derivatives y=(100^c - (x-100)^c)^(1/c). c=1 gives a straight line, c=2 gives the usual circle shape.

Just wondering, is the graph somehow related to finding a signal with a lot of background?