Scatter plots are 'noisy' with Smoothed Line OFF

**nmpixel** · 03-12-2018, 04:14 AM

Warning - I posted this question at another site but could not upload photos there or edit. Trying here instead as the question is made easier with photos.

Can somebody help with a strange Excel behavior related to graphing? The basic question: why do scatter plots appear rough or noisy for simple functions like y=sin(x) or y = mx+b when the ‘Smoothed Line’ option is OFF?

The screenshots show basic functions such as y = sin(x) or y = 0.8x + 0.5 (line width = 0, markers removed). The roughness or ‘noise’ is easily visible. The roughness disappears completely when the Smoothed Line option is switched ON, but why is there such a high degree of roughness when the option is OFF? I have verified this on 3 different computers (also PC and Mac) with different Excel versions.

At first I thought this was due to the IEE 754 floating point standard that limits Excel to 15 digits. But now I’m not so sure, because the degree of roughness in these graphs would represent error in the 5th, 6th, or 7th decimal place, not the 15th.

If interested, try this quick test. Create a column of x values 0, 0.02, 0.04 and another with sin(x). Then plot a scatter plot with Smoothed Line OFF and zoom in close. You should see this same jitter or noise.

Thanks!

Sin(x) and mx+b (with Smoothing OFF)
Screen Shot 2018-03-11 at 8.51.20 PM.png

two equations for half circle (with Smoothing OFF)
Screen Shot 2018-03-11 at 10.00.59 PM.png

two equations for half circle (with Smoothing ON)
Screen Shot 2018-03-11 at 7.49.20 PM.png

**AliGW** · 03-12-2018, 10:09 AM

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**nmpixel** · 03-12-2018, 10:37 AM

I have less than 10 posts and cannot post a link. Therefore, I issued this announcement at the very top of my post:

Warning - I posted this question at another site but could not upload photos there or edit.

The other post is on Mr. Excel with exactly the same post title (for transparency). I double posted because photos are critical to describe this problem, and I could not easily upload photos or edit the post on MrExcel.

**nmpixel** · 03-13-2018, 05:42 PM

important correction - an x increment larger than 0.02 is too coarse to see the jitter. This screenshot shows a 0.002 increment. What a strange behavior.
SIN_LINE_jitter_Excel2.png