History:

I created a spreadsheet with functions only that will provide feedback from
a data point on an xy scatter chart. I charted several thousand cells with
simple functions such that =if(and(a,b),1,0) to provide feedback based on
these coordinates and then used v or h lookup to come up with the adjusted
information. This is for a field determination of what the data points should
produce.

I thought it would be nice to duplicate the form for a similar process in
the lab. We frequently test the field information to gain more exact
information in the lab. This would allow me to show both the field
determination and the lab results on the same chart.

The lab test is performed and results in 3 to 8 x,y data points. There will
never be less than 3 or more than 8 even if it results in 400 samples. We
will only use the last 8 at the maximum. Through testing I have found that
the 2nd order polynomial trend line exactly pinpoints the results 99.9% of
the time,(I haven't had a miss yet but you never know). I need to know how to
draw the x,y data point from the apex of the polynomial trend line.

Example:

x = 13.0,15.1,16.9,19.2
y = 120.0,125.0,130.0,127.0

X will always grow no matter how many tests we perform(at a relatively
stable rate). Y grows and breaks, declines, at a very unstable rate. Y will
always break 1 data point prior to the last. All points are calculated to the
nearest tenth (#.#)

I have set the series up such that if the y point breaks, the remainder of
the empty x and y cells are automatically set to the final break point
(127,19 see above). This seems to eliminate all the empty data points by
merging them into the last and subsequently eliminates them from the
polynomial trend line.

Question

How do I find the apex of this polynomial trend line? I ran several hundred
samples and have found that the trend line equation that excel emits is
always different. I tried running the regression analysis but found no two
cells that could be used as some sort of factoring.

If this is a VBA answer and you choose to help me, please feed me pablum as
I have no clue how to even begin with VBA. I do know javascript fairly well
if there is any coorelation.