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Re: Controling the color of line segments in a Log plot

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  • Subject: [mg90171] Re: Controling the color of line segments in a Log plot
  • From: Szabolcs Horvát <szhorvat at>
  • Date: Tue, 1 Jul 2008 07:00:30 -0400 (EDT)
  • References: <g4a6r8$8s5$>

Donald DuBois wrote:
> I would like to control the color of line segments in a Log plot. I have three Questions (A,B,C) regarding this problem.  
> A simple example:  I start with the data yValues = Table[10^n, {n,10}] - which should also be the Y-axis tick values - and use ListLogPlot to graph it.
> yValues = Table[10^n, {n, 10}]
> xy = Transpose[{Range[Length[yValues]], yValues}]
> ListLogPlot[xy, Joined -> True]
> Question A . Does anyone find it peculiar that ListLogPlot seems to be taking 
> Log to base 10 as a default as in the above plot when Log by itself defaults to base E?  This is FINE for me, as I want it to be to the base 10. But still, 
> does anyone know why this is so or am I misconstruing something?

It does not use base 10.  The tick labels are the actual values, not the 
logarithms (i.e. it uses 10^6 and not 6).

> Question B. 
> I would like to change the color to Red of the line segment with X values going from
> 3 to 6. (In general, I would like to change the segment color to Red for any number of segments of the same Log plot.) Is there a way to do this at the  ListLogPlot level ( directives directly to ListLogPlot)?
> That would be the simplest solution. 

Yes, it is possible with Mesh and related options.

ListLogPlot[xy, Joined -> True, Mesh -> {{3, 6}},
  MeshShading -> {Blue, Red, Blue}(*,MeshStyle->None*)]

Check the doc page of the Mesh* options, and also the examples related 
to these options on the doc pages of various plotting functions.  (They 
were not completely intuitive to me until I checked the examples.)

> I can change the color of this part of the graph by going down to the Graphics element level using Line as in the following, but this is tedious.
> logY = Map[N[ Log[10, #]] &, yValues]
> xyLog = Transpose[{Range[Length[logY]], logY}]
> l2 = Graphics[{Red, Thick, Line[Take[xyLog, {3, 6}]]}];
> l1 =  Graphics[{Line[Take[xyLog, {1, 3}]]}]; 
> l3 =   Graphics[{Line[Take[xyLog, {6, 10}]]}];
> Show[l1, l2, l3, Axes -> True, AxesOrigin -> {0, 0}]
> Question C.  The above graph loses the original Y axis tick values which are contained in the yValues list defined above.  Is there any way to solve this?  I want the graph to look like the first graph above  with the same Y tick values (using ListLogPlot) except the Line segment with X values from 3 to 6 should be Red.

Well, maybe this is little cheating but, I think that the simplest 
solution would be to extract the tick specifications from the original 
graphic and use them in the Show function above:

gr = ListLogPlot[xy, Joined -> True]
myLogTicks = Ticks /. AbsoluteOptions[gr, Ticks]

Show[ ..., Ticks -> myLogTicks]

(This may need some adjustments before it will look right if you use a 
different plot range in Show than in gr.)

> Thank you in advance for any help you can give me.
> Don

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