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Possible Duplicate:
How to insert guide-lines in graphics without specifying range?

How to plot multiple horizontal lines?

For instance, I want to plot 3 horizontal lines: y=-1, y=0 and y=1. Seems to be a pretty simple thing, but I couldn't figure out how; although I've searched all over the documentation center.

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marked as duplicate by Artes, Mr.Wizard Dec 7 '12 at 10:33

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

check out GridLines with some of the examples of gridlines with specific values. – Mike Honeychurch Dec 7 '12 at 5:16
Thanks! I'll try. – Lei Chen Dec 7 '12 at 5:19
@LeiChen Can you add more detail to your question? Do you want a graphic only with lines or do you want the lines in a e.g. plot of a function as grid? – halirutan Dec 7 '12 at 5:30
@halirutan I just want multiple horizontal lines. How to plot Function with grid is well explained by the documentation center. Thanks! – Lei Chen Dec 7 '12 at 5:35
Drawing lines is also pretty well covered in the manual. What was the problem you faced there? – Sjoerd C. de Vries Dec 7 '12 at 6:59

If you just want to draw Lines, than you can create your own Graphics. I hope the code is self-explanatory

  Line[{{0, -1}, {1, -1}}],
  Line[{{0, 0}, {1, 0}}],
  Line[{{0, 1}, {1, 1}}]

Mathematica graphics

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Thanks!! You just solved something I wasn't able to figure out a whole night! – Lei Chen Dec 7 '12 at 5:44
@LeiChen Then please take some minutes to read the FAQ. Especially, you should understand that we have a voting system here and the -1 infront of your question means in your case: "your question is not explained in detail so that it is not clear what you mean". You can always edit your question and you should learn what up/downvoting and accepting is! – halirutan Dec 7 '12 at 5:47
even that is easier to do with gridLines -- Graphics[{}, GridLines -> {None, {-1, 0, 1}}] – Mike Honeychurch Dec 7 '12 at 5:51

The long way round:

  Evaluate[y = -1],
  Evaluate[y = 0],
  Evaluate[y = 1]
 }, {x, -10, 10}, PlotStyle -> {Thick}, PlotRange -> {-10, 10}]

the plot thickens

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Why not just Plot[{-1, 0, 1}, ...]? – Rahul Dec 7 '12 at 15:57
:) I was trying to get as close to the question as possible... – cormullion Dec 7 '12 at 16:07

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