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MathGroup Archive 2007

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Re: Spurious Line

  • To: mathgroup at smc.vnet.net
  • Subject: [mg75049] Re: Spurious Line
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sun, 15 Apr 2007 05:12:11 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <evpoeu$69p$1@smc.vnet.net>

Jeff Albert wrote:
> When I run the following I get what is obviously a spurious straight line
> connecting (-100,0) and (300,400*.7).  How do I get rid of that line?
> 
> {{-100,0},{0,-100},{100,-200*.9},{200,-300*.8},{300,-400*.7},{-100,0},{0,100
> },{100,200*.9},{200,300*.8},{300,400*.7}}
> 
> ListPlot[%, PlotJoined->True]
> 
> Jeff

Hi Jeff,

There is no spurious line in the plot, indeed. The option PlotJoined 
draws a line between each pair of points. The pairs are a partition of 
the list of points with an offset of one. First, a line is drawn between 
points 1 and 2, then a line is drawn between points 2 and 3, and so on. 
Evaluating the following expressions will show a step by step building 
of the plot and the creation of the incriminated line.

In[1]:=
list = {{-100, 0}, {0, -100}, {100, -200*0.9},
     {200, -300*0.8}, {300, -400*0.7}, {-100, 0},
     {0, 100}, {100, 200*0.9}, {200, 300*0.8},
     {300, 400*0.7}};

Needs["Graphics`Graphics`"];

g = (ListPlot[##1, PlotJoined -> True, Frame -> True,
       DisplayFunction -> Identity, PlotRange ->
        {{Min[list[[All,1]]], Max[list[[All,1]]]},
         {Min[list[[All,2]]], Max[list[[All,2]]]}},
       PlotLabel -> StringJoin["Drawing line from ",
         ToString[First[##1]], " to ",
         ToString[Last[##1]]], PlotStyle ->
        {Red, AbsoluteThickness[3]}] & ) /@
     Partition[list, 2, 1];

Do[DisplayTogether[g[[Range[n]]]], {n, 1, Length[g]}]

Regards,
Jean-Marc





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