Yahya -

You might get a good fit using a logistic function. Use Google to search for
"excel logistic curve" (without the quote marks).

But, when possible, curve fitting should rely on knowledge about the
physical phenomenon that is being modeled. Please share what you know about
the source of the data. Such knowledge is usually important for selecting an
appropriate functional form.

- Mike

http://www.MikeMiddleton.com



"Yahya" wrote in message
...


"Jon Peltier" wrote:


"Ed Ferrero" wrote in message
...
Hi Yahya,

I have a chart in Excel 2007 and I want to provide a Y value (that is
not
one
of the data points) and have Excel to tell me the corresponding X
value
from
the chart. How can I do that?
The chart is not a straight line, so trendline won't work
Here is a picture of the chart I'm working on
http://hkoyda.blu.livefilestore.com/...x_wA/graph.JPG

Any help in this matter would be greatly appreciated.
Yahya

You can't do that without knowing the formula for your line of best
fit.

There is a sample here
http://www.edferrero.com/ExcelCharts...2/Default.aspx called
'Reading Interpolated Values' that works for a straight line. The
sample
uses click-and-drag on the chart which will not work in Excel 2007.
However, you can manually change the x-Value at cell E4 to calculate
the
intercept at a particular x-Value.

Looking at your picture, I would try calculating the log of the
y-Values
and charting that. If this is close enough to a straight line for your
purposes, then just use my sample to calculate the intercept. =LN() to
find the log, =EXP() to change back.


If the coordinates of the points are known, i.e., the values are in the
worksheet, then a stepwise interpolation approach can be implemented.

In general, if these are measured points, unless you know the form of the
relationship, you should refrain from using smoothed lines to connect the
points. Excel uses an arbitrary algorithm for smoothing the lines, which
has
no physical bearing on the chart, and which may cause the smoothed line
to
deviate substantially from a well-behaved relationship.

- Jon
-------
Jon Peltier, Microsoft Excel MVP
Peltier Technical Services, Inc.
http://PeltierTech.com/WordPress/
_______



Yeah I have the points coordinates, but I don't know how to do an
interpolation
Here is a snapshot of my data points
http://hkoyda.blu.livefilestore.com/...VDOA/graph.JPG