How to draw line vertical to Y-axis?

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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.
Thanks so much in advance,

Accepted Answer

Walter Roberson
Walter Roberson on 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');
Gourav Mitra
Gourav Mitra on 12 Apr 2021
it works fine but in some case, the vertical line doesnot fit or cover the entire ylimit.
Walter Roberson
Walter Roberson on 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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More Answers (5)

Noushin Farnoud
Noushin Farnoud on 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:
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.

Eric Sargent
Eric Sargent on 15 Oct 2019
Starting in R2018b you can simply use xline and yline.
In this case:

Sven on 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')
Sven on 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 on 28 Mar 2014
There is an excellent answer over on 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]);
%# horizontal line
hy = graph2d.constantline(0, 'Color',[.7 .7 .7]);

Wayne King
Wayne King on 2 Nov 2011
a = [1 2 3 4 5 6 7];
b = [1 4 7 10 7 4 1];
hold on
  1 Comment
Walter Roberson
Walter Roberson on 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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