I'm struggling to understand what your concern is.

First, with your formulas in column K, I do not get any errors if I copy J27:J28 and J29 and copy C27 into C28:C29. And the resulting yellow line

__closely__ tracks the chart trendline. (I am using Excel 2010.)

However, you are using a Line chart instead of an XY chart, and the trendline of a Line chart is known to be incorrect unless the x-axis values are 1, 2, 3, etc. See

https://support.microsoft.com/en-us/help/211967/chart-trendline-formula-is-inaccurate-in-excel .

The Line chart trendline uses 1, 2, 3 etc for the x-axis, which are equidistant apart, not your data. Your LINEST formulas use the actual dates in column A; they are the numeric values 41395, 41426, 41456 etc, which are not equidistant apart.

(To make matters worse, the "dates" in C28:C29 include times of day. They should not.)

If you substitute 1, 2, 3 etc in column A, the values calculated in column J (and C) fit the chart trendline more closely, if not exactly.

Nevertheless, the difference is relatively small in this case.

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In general, it is unwise to extrapolate an polynomial trendline (or LINEST equivalent), unless the actual data is "polynomial" by its nature (rarely the case). Polynomial trendlines should be used only to

*interpolate* between the endpoints of the original data.

This is demonstrated by the extreme drop-off of your polynomial trendline. The fact that it seems to follow the data for Mar'15 is coincidence, if not artificial. ( I cannot imagine any reason why the actual data for Mar'15 would experience a drop of more than 83%.)

In any case, that is why the predicted (trendline) value for May'15 is negative, if that is your concern.

(In fact, the chart trendline also appears to go negative for May'15.)

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PS.... Your use of LINEST can be simplified. Select F5:I5 and

*array-enter* (press

*ctrl+shift+Enter* instead of just Enter) the following formula:

=LINEST(B5:B27,A5:A27^{1,2,3})

Moreover, the formulas in column J can be of the form:

=SERIESSUM(A5,3,-1,$J$5:$I$5)