Trendline vs. equation

[Phil Hageman]

I am plotting a series against the secondary axis (right
side of chart), with an included second order polynomial
trendline. Also, the following formula is calculates (I
thought) where the trendline intersects the "Y" secondary)
axis.
{=LINEST(G14:G185,B14:B185^{1,2})}
G series data is value
B series data is month/year

I should think both values equal; however, they do not
agree (for the month ending August 03):
Fourmula $293,744
Trendline $365,000 (at the trendline/axis intersect)

Is there a way to accurately calculate the actual value
where the trendline crosses the "Y" axis?

 

[Phil Hageman]

Jerry, thanks very much for your time and response. The
series is plotted as a line chart where the trend line
crosses at about $360,000. I made a scattergram, using
the same X and Y range values, converted the dates, and
added a second order polynomial trendline - resulting in
an intersect at about $320,000. Obviously a change. Can
you help me with the equation/formula that gives the exact
value of the intersect in the scattergram? And, which
chart (line or scattergram) developes a more accurate
trendline? The $40,000 (11%) variance brings both graph
systems into question as to accuracy. Or am I using them
incorrectly?
Thanks, Phil

 

[Jerry W. Lewis]

Not in general. As I mentioned previously, the formula displayed on a
Line chart totally ignores your x-data, replacing it instead with 1, 2,
3, ...

Jerry