As an appraiser I use Excel to analyze market data and to find the
value of amenities in a given market.

When analyzing market data, I use selling date(X) vs SqFt(Y) to create
a chart. I then create a trendline to present what are sometimes 200-
300 data points into a representation that can be easily understood by
a reader. I usually use a 3-4 order polynomial trendline. In the name
of clarity I can and sometimes do render the data points invisible,
leaving just the trendline.

My question is this: How do I at this point insert along the trend line
values for Y wherever the trendline crosses a point on X?

This would be helpful for figuring the "average value" for Y on any
given date.

Bill McKnight, Appraisal Ace, Citrus Heights, CA