Extracting multiple points from a chart

[mpearson4]

Hi,

I am wondering if there is a way to extract multiple data points from a chart. Hopefully I have successfully managed to attach an example to this post...

Basically I have a scatterplot with thousands of points. I want to be able to select a bunch of points, in this case the ones in yellow and extract the data for just those points. My example shows a riverbed with the y axis being water depth and the x axis being distance from a specific point. I want to be able to go through and select all the lowest points and extract that data so that for those points only I have distance and depth. Does that make sense?

Thanks in advance
Marita

[Glenn Kennedy]

Hi there... and welcome to the Excel Forum. You have attached an image. That's impossible to work with. We have to re-type your information before we can begin to address your problem, and can't even do that on this occasion - 'cos we can't see the data! That will put a lot of people off completely. Not only that, but (depending on their choice of browser) many members cannot even see the image.

Please attach a sample workbook. Make sure there is enough data to demonstrate your need. Make sure your desired results are shown, mock them up manually if necessary. Remember to remove ALL confidential information first!!!

However, nothing on this Forum works quite as well as you might expect. The attachment icon doesn't work!! (Helpful, isn't it!!). Instead, just before you submit your post, click on GO ADVANCED (near the bottom) and then scroll down to Manage Attachments to open the upload window. The relevant instructions are at the top of that screen.

[mpearson4]

I have attached a work book. Hopefully this one is clear. THANKS

[MrShorty]

The chart itself will not really help do this. Charts really only visualize the analysis done in the spreadsheet.

As I noted a couple of days ago in a similar question, this kind of signal analysis can get fairly involved. Generic approach that I would expect to use on this:

1) From calculus, I learned that minima/extrema tend to occur where the slope of a function is 0 or changes sign. So, one of the first things I usually do is to add a column to compute slope. (y2-y1)/(x2-x1) is usually good enough for noise free data, though I may choose different variations depending on noise and other considerations.
2) Then, a function to locate sign changes in the slope column. Something like =sign(a2)=sign(a1) will return TRUE if the signs are the same and FALSE if not.
3) The points you highlighted in your picture suggest that you are not looking for every minimum, but only certain minimum. I will usually add different columns for the different criteria that I will use to identify minima that I want to capture. This is often the most difficult part, because I have to think very carefully through the criteria I want to use (and how those criteria all combine together) to correctly identify the desired minima.
4) The result of all of that is usually a lookup column that I can apply a lookup function to (or a filter or similar) to extract the desired points. Can I assume you are familiar with Excel's lookup functions?

Step 3 is the one that is least clear to me, and you will need to clarify that step. Are you able to formulate the other steps? Where do you get stuck?