Need help with adding a +10% and -10% line above and below a given trendline

I am working on some comparative data analysis involving two columns of data (original vs. duplicate assay results) that are being displayed on a scatter plot graph. The trend line that I add to the scatter plot data is useful but I would also like to display a +10% and -10% "warning line" above and below the trend line.

Like this:
ScatterPlot_TrendLine-with-WarningLines.PNG

Has anyone done this before? I obtained the image from some company literature, however, it does not describe how to accomplish the graph shown.

Can I display a simple line that uses the same linear formula with a slight adjustment to the slope (+ or - 10% to the slope value)?

Please help.

The picture given is not real data, or at least I don't have the data or program or file that that came from. It's just an example of what I'm trying to achieve.

Thanks for your reply. I can attach the file but I will need help on how to create two new rows/columns that would represent + or - 10% of the trend line.

I'm using Excel 2010, so most of these functions are "user friendly" (ie. I can see the guts of how they work). After creating the scatter plot graph I right clicked on the data and chose "Add Trendline...". And under "Format Trendline" there aren't too many options.

I also tried a version of what you are suggesting by taking the X axis data and the Y axis data and independently increased or decreased the data by 10%. But when this is plotted it only artificially augments or decreases the original data set (therefore a trendline generated on the newly augmented or decreased data would yield the same trendline).

This is what I've come up with based on your suggestion:
2ndAttemptControlLines.PNG

What's weird now is that when you run a percent difference on the original trend line slope value against the two new trend line slope values you get percent differences of 22.2% and 18.2% (respectively for the "plus 10" and the "minus 10").

In addition to that I don't want to have to show the fictitious data that was used to generate the two new trend lines. How do I remove that data and still display the +10 and -10 lines?

See attached where I re-created the chart.

Your 10% is not 10% of the trendline, it's 10% of your data. Not what you want. You can add a trendline that Excel plots automatically but then you don't have the data for it. SO you have to use the TREND function to be able to calculate the +/-10%. All shown in the attachment.

Thanks for showing me how the TREND function works. It really demystified the trendline function in Excel. And after doing more research into my company's guideline representation of this kind of scatter plot data (as show above in the first example). The "trend line" is in fact not a trend line (like you seemed to catch onto from the first reply). It is just a y=x line, with the +/- 10% generated the same way as you have done but with the y=x line as a reference.

The result is in my opinion cheating. Or an over simplification of the data, using an idealized 1:1 ratio of the comparative data. Which doesn't really tell you very much about your actual data, just how it fits in with an ideal situation.

Thanks again. I will have to play around with the TREND function. I somewhat understand the syntax but I will need to study it a bit more to understand it more fully.

After your explanation, I'm not sure what you really need--a reference line, or a true trend line. If you are measuring tolerances from a standard then you want the x=y line instead of the TREND data.

TREND looks at your data give by the x values and y values given in the first two arguments and calculates a trend line. Then the third argument is an x value, and it determines the corresponding y value that falls on the trend line. Related functions for analyzing variability of data are LINEST, INTERCEPT, SLOPE, RSQ.