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Why does a Calc square root plot have a kink? [closed]

asked 2017-04-24 18:41:03 +0200

David.Cheshire gravatar image

If I make a linear sequence of numbers e.g. from 100 to zero step -10, then take the square root of each number and make a chart of these square roots, it has a kink around 9 and 10. I can't upload an example sorry, but here is the data:

Sequence Sqrt() 100 10 90 9.48683298050514 80 8.94427190999916 70 8.36660026534076 60 7.74596669241483 50 7.07106781186548 40 6.32455532033676 30 5.47722557505166 20 4.47213595499958 10 3.16227766016838 0 0

The chart of the Sqrt() column has a downward kink at 9 on the x axis, followed by an upward kink before 10. Why is this?

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Closed for the following reason question is not relevant or outdated by Alex Kemp
close date 2020-10-01 13:43:13.624453

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Hello David Cheshire, i could not reproduce your problem in Calc Version: 5.1.6.2, plz specify which Chart Type produces the kink

librebel gravatar imagelibrebel ( 2017-04-24 19:35:40 +0200 )edit

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answered 2017-04-25 01:22:10 +0200

Lupp gravatar image

updated 2017-04-25 01:32:52 +0200

I suppose the chart was "x-y (Scatter)" what is the common way to visualize mathematical functions..
You may have chosen 'Straight' for the line type, but expect it 'Smooth' nonetheless?

Please note: The diagram of the SQRT function in mathematical mode is a half parabola (divided by the axis of symmetry). The curvature of a parabola has a prominent maximum at its vertex. Even if you choose smooth line type, the chart may look somehow "kinky" if the resolution of the x-axis is not adequate. 10 steps for all the graph is simply too coarse if you put the diagram in a "normal" size, and look at it from a "normal" distance.
See also this demo.

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Hi Lupp, thanks for your answer. It makes no difference whether I choose a normal line or an xy scatter, and I had selected smooth rather than straight, but you are correct, the step size was too big. When I use smaller steps the curve is smoother. Which is something I didn't expect, I anticipated that the curve smoothing algorithm would produce a smooth curve, but evidently it doesn't. Something to remember for sparse data sets!

David.Cheshire gravatar imageDavid.Cheshire ( 2017-04-28 12:25:09 +0200 )edit

Quoting @David.Cheshire: "It makes no difference whether I choose a normal line or an xy scatter..."
If you want reasonable lines the 'Scatter' type is the only one usable. The 'Line' type you call "normal" is for misuse by those not understanding charts - or for deception. Only in the case of equidistant x values it makes sense and does then produce the same result as the 'Scatter' type.

Lupp gravatar imageLupp ( 2017-04-28 13:14:50 +0200 )edit

Actually the curve you get with option 'Smooth' is smooth. However, the algorith is fortunately not from AI labs. It cannot know therefore that you wanted the line to be a half parabola. For someone having the parabola in mind the result is not satisfactory. But there are many types of curves in the world.

Lupp gravatar imageLupp ( 2017-04-28 13:17:29 +0200 )edit

Hi Lupp, thanks, in this case the x values were equidistant so I could use either a line or scatter chart. Is it possible to change the default chart settings?

David.Cheshire gravatar imageDavid.Cheshire ( 2017-04-28 13:20:16 +0200 )edit

" Is it possible to change the default chart settings?"
I don't know a way, and most likely there is none.

Lupp gravatar imageLupp ( 2017-04-28 13:23:29 +0200 )edit

OK, thanks.

David.Cheshire gravatar imageDavid.Cheshire ( 2017-04-28 16:08:38 +0200 )edit

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Asked: 2017-04-24 18:41:03 +0200

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Last updated: Apr 25 '17