# How to draw line vertical to Y-axis?

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M G il 2 Nov 2011
Commentato: Walter Roberson il 12 Apr 2021
Hey Matlab users,
If a = [1 2 3 4 5 6 7]; and b = [1 4 7 10 7 4 1]; and the plot it by : figure,plot(a,b), how can I draw let's say a red line vertical to the value 4 and 7 in the y-axis (parallel to x-axis)? So that i can see the window between 4 and 7.
Mehdi
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### Risposta accettata

Walter Roberson il 2 Nov 2011
The same as for your previous question. The solution for that was
yL = get(gca,'YLim');
line([3 3],yL,'Color','r');
line([5 5],yL,'Color','r');
and if you are wanting to draw vertical lines at 4 and 7, you would just change the code to
yL = get(gca,'YLim');
line([4 4],yL,'Color','r');
line([7 7],yL,'Color','r');
Perhaps what you want is instead horizontal lines? If so then
xL = get(gca,'XLim');
line(xL,[4 4],'Color','r');
line(xL,[7 7],'Color','r');
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Gourav Mitra il 12 Apr 2021
it works fine but in some case, the vertical line doesnot fit or cover the entire ylimit.
Walter Roberson il 12 Apr 2021
These days, see yline()
If you get the current ylim and use it to draw, and then you zoom or pan, then the code I posted back then will not adjust to draw according to the new limits. However, the newer yline() should do that.

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### Più risposte (5)

Noushin Farnoud il 15 Nov 2011
If you want to draw a line that spans the entire xlim or ylim of your figure, then I suggest that you use hline or vline functions from: http://www.mathworks.com/matlabcentral/fileexchange/1039-hline-and-vline
You can also plot multiple vertical or horizontal lines at the same time using these functions. For example: hline([5,6,8]) draws 3 horizontal lines at x=5, x=6 and x=8.
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Eric Sargent il 15 Ott 2019
Starting in R2018b you can simply use xline and yline.
In this case:
yline(4,'r');
yline(7,'r');
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Sven il 2 Nov 2011
Hi Mehdi, try this after plotting:
xlims = get(gca,'XLim');
hold on
plot(xlims, [4 4], '-r')
plot(xlims, [7 7], '-r')
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Walter Roberson il 2 Nov 2011
Confusing...
Sven il 2 Nov 2011
Doh! I voted yours up and now it's above mine... that's a blow to my chances for getting accepted! heheh

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James il 28 Mar 2014
There is an excellent answer over on http://stackoverflow.com/a/8108766/1194420 repeated below for convenience. ---
There exist an undocumented function graph2d.constantline:
plot(-2:5, (-2:5).^2-1)
%# vertical line
hx = graph2d.constantline(0, 'LineStyle',':', 'Color',[.7 .7 .7]);
changedependvar(hx,'x');
%# horizontal line
hy = graph2d.constantline(0, 'Color',[.7 .7 .7]);
changedependvar(hy,'y');
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Wayne King il 2 Nov 2011
a = [1 2 3 4 5 6 7];
b = [1 4 7 10 7 4 1];
plot(a,b)
hold on
plot(a,4*ones(length(b)),'r');
plot(a,7*ones(length(b)),'r');
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Walter Roberson il 2 Nov 2011
A bit murky if "a" does not happen to be in sorted order, especially if the drawingmode is set to xor

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