I have very little to add to Mike Middleton's comprehensive reply,
except to note that

- for Mike's comment (4), the simplest approach is to format in
scientific notation with 14 decimal places.

- If the chart is an "XY (Scatter)" chart with exclusively numeric X
data (cf. Mike's comments 2 & 3) and the coefficients still differ
significantly from LINEST, then the fitting problem is ill-conditioned
and the chart coefficients are more accurate.

Jerry

Mike Middleton wrote:

Simon -

Several comments:

(1) A 3rd order polynomial may be overfitting your data. It may fit the
original data well, but it may not be useful for predicting future values.

(2) If you're using a Line chart type instead of an XY (Scatter) chart type,
Excel is using the values 1,2,3,... for X.

(3) If you're using an XY (Scatter) chart type and if any of the X data is
text instead of numerical, Excel may be using 1,2,3,... for X.

(4) Select the trendline equation and repeatedly click the Increase Decimal
button to display more significant digits.

(5) To obtain the coefficients in worksheet cells, see the "Regression with
polynomials" section at

http://www.tushar-mehta.com/excel/ti...efficients.htm

- Mike
www.mikemiddleton.com

"Simon Y" <Simon wrote in message
...

I am trying to establish the polynomial coefffiecients for a fuel
consumtption curve for one of our ships. I have plotted the recorded
consumption data from the ship and trended them with a 3rd power
polynomial. the curve folllows the data well, however, the set of
genertated coefficients, which when put back into the graph to generte
test
curve that in no way refelct the original tend line. and can't be used to
forcatst the fuel budget. I must be doing something stupid.
Thanks,
Simon.