I have 2 series (x,y) of 11 numbers than I can scatter plot and insert a
trend line on. My problem is that I have a 12th x value with no corresponding
y value. how can I find the y value in the trend? and put it on the graph?

Graphically, I need to extend a verticle line up from the 12th x axis
coordinate up to the intersection with the trend line from the first 11.

Thanks