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Groups > comp.soft-sys.math.mathematica > #1673

Undesired horizontal line in Plot / Bug?

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A phenomenon that occasionally also occurs in other plots
can be exemplified by setting

fD[n_, cZ_] := 137^2/Sqrt[1 + cZ^2/(137*n + Sqrt[137^2 - cZ^2])^2];

and plotting, e.g., 

Plot[{fD[1, z], fD[5, z]}, {z, 1, 137}, Frame -> True, 
 PlotRange -> Automatic]

There is a horizonal line visible (parallel to the abscissa)
that should not be there!! In fact, other plots like, e.g.,

Plot[{fD[1, z], fD[2, z]}, {z, 1, 137}, Frame -> True, 
 PlotRange -> Automatic]

look fine, without this annoying additional line. Unfortunately,
I could not find a way to remove or suppress this unwanted
horizontal line in the first plot. Moreover, I observed this 
effect in all versions of Mathematica I have access to at the
moment, i.e., four, six, and seven, and it makes me wonder
whether this is a yet unresolved bug.

Thus I would be very grateful for any hints on the questions

  - whether this horizontal line also occurs in the
    current Mathematica 8.0 ?
  - is there any remedy to prevent the line from being
    plotted or to remove it from the plot?

Many thanks in advance for your kind help, H. Hogreve

P.S.: If plotted without Frame, then this line does not
 occur, but apparently at its position the new abscissa
 is placed

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Thread

Undesired horizontal line in Plot / Bug? "H Hogreve" <hogreve@ifisr.org> - 2011-04-14 08:58 +0000
  Re: Undesired horizontal line in Plot / Bug? Albert Retey <awnl@gmx-topmail.de> - 2011-04-14 11:08 +0000
    Re: Undesired horizontal line in Plot / Bug? "Kevin J. McCann" <Kevin.McCann@umbc.edu> - 2011-04-15 07:54 +0000

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