How does Prism compute the value of X at Y mid and the value of Y at Xmid?

 When you enter a new user-defined equation into Prism, you also create rules to generate initial values for each parameter. These rules can use computed values like Ymin (smallest Y value), Ymid (the average of the largest and smallest Y value), etc. Two of the choices are "Value of X at Ymid" and "Value of Y at X mid". 

To compute the value of Y at Xmid, Prism follows these steps:

1. Compute Xmid, which is the mean of the largest and smallest X values. 
2. Of all the data points whose X value is smaller than Xmid, find the one with the largest X value.
3. Of all the points whose X value is larger than Xmid, find the one with the smallest X value. 
4. Draw a straight line between those two points.
5. Calculate the value of Y  on that line when X equals Xmid. 

The value of X at Ymid is computed in a similar way.