Andy]]>

1. First you create a point series with your data points.

2. Select the point series and choose *Function|Insert trendline* in the menu.

3. In the *User defined* tab, you can specify your model as **$a*sin(atan(-$b*x))**

4. You can optionally use the *Add model* button to store the model for later use. It also allows you to change the default guess, which is sometimes needed.

5. When you click the *OK* button, Graph will create a function from your model that fits your point series.

f(x) = a {sin[atan(-bx]}

and a set of points. I determined a and b by trial and error to make the curve match the points. Is there a better method to let the software do the work?]]>