I have data that I believe might fit some kind of semi-sinusoidal trend line - I'm trying to derive an equation for the length of individual vertebrae in various species (I am not a mathematician). I'd like to put that trend line in Excel - it is not essential for the trend line to fit the error bars, just the data for now.
Here is my graph for the Human Spine (the Unit Height is the height of the bone and the Inter-vertebral Disk):
