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Philip on 25 Feb 2011
Hi there, Can anyone please tell me how I can add a vertical line to my plot at a specified sample point? For example, I have a a 1x41 vector of intensity values, and I would like to add a vertical line on the center sample (sample number 21). Many thanks!
Paulo Silva on 25 Feb 2011
fig=figure;
hax=axes;
x=0:0.1:10;
hold on
plot(x,sin(x))
line([SP SP],get(hax,'YLim'),'Color',[1 0 0])

Michelle Hirsch on 28 Apr 2022
Edited: MathWorks Support Team on 28 Apr 2022
Woohoo - this is built into MATLAB now, as of R2018b!
If you are running R2018b or later, you can use the “xline” and “yline” functions. For example, create a vertical line at x=5:
xline(5)
Create a horizontal line at y=10:
yline(10)
Starting in R2021a, you can create multiple horizontal or vertical lines in one pass. For example, create vertical lines at x=1, x=2, and x=3:
xline([1 2 3])
If you are running R2018a or earlier, use the “plot” function with this pattern:
Horizontal line:
plot([x1 x2],[y y])
Vertical line:
plot([x x],[y1 y2])
For example, plot a vertical line at x = 21. Set the y values using the y-axis limits of the axes.
y = ylim; % current y-axis limits
plot([21 21],[y(1) y(2)])
Roberto Chang on 23 Aug 2021
Hello all! do you know if this magical (awesome) feature can be done in Z axis for bar3?

line([x x], [y1 y2]); is the easy command;
Christian Sanchez on 8 May 2020
Genial

carolina franco on 26 Oct 2017
Edited: MathWorks Support Team on 8 Nov 2018
You can plot a horizontal or vertical line using the “plot” function with this pattern:
- Horizontal line:
plot([x1 x2],[y y])
- Vertical line:
plot([x x],[y1 y2])
For example, plot a vertical line at x = 21. Set the y values using the y-axis limits of the axes.
y = ylim; % current y-axis limits
plot([21 21],[y(1) y(2)])
As Steven suggested, starting in R2018b, you can use the “xline” and “yline” functions instead. For more information, see:
Rasmus Ringsborg Nielsen on 11 Mar 2021
Thank you so much, works perfect!!

Mark on 12 Mar 2013
Edited: Mark on 12 Mar 2013
Probably the simplest way:
Choose the x-value where you want the line "xval." Choose the minimum y value to be displayed on your graph "ymin" and the maximum y value to be displayed on your graph "ymax."
x=[xval,xval];
y=[ymin,ymax];
plot(x,y)
Flaws with this method: probably will look silly if you use '-x' or '-.', these mark your specific points on the line, but you'll only have two (at least they're endpoints).

Steven Lord on 1 Nov 2018
If you're using release R2018b or later, use the xline or yline functions to create lines with constant x or y values respectively.
Gary Bikini on 26 Apr 2019

the cyclist on 25 Feb 2011
One way:
figure
x = rand(1,41);
y = 1:41;
plot(x,y,'r.');
line([x(21) x(21)],[0 41]);
set(gca,'YLim',[0 41])

James on 28 Mar 2014
Edited: James on 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');
Ben on 9 Sep 2016
@Steven That's because undocumented features can be removed at any time, as this feature was.

Maybe it is a bit late but I want to contribute, there is a really easy way to add vertical and horizontal lines, you just have to use a hold and then overlap them over the main plot.
Before declaring the original plot, add a hold on to ensure it will retain both plots, then plot the lines, with this structure:
hold on;
plot(the main function)
plot([x x],[0 y_max]) % Vertical Line
plot([o x_max],[y y]) % Horizontal line
Being:
x: location on horizontal axis where you place the vertical line.
y: location on vertical axis where you place the horizontal line.
x_max: point where you want the vertical line to end.
y_max: point where you want the horizontal line to end.
Thanks!

Jos (10584) on 8 Jul 2014
You might also be interested in GRIDXY on the File Exchange:

Julian Williams on 9 Feb 2019
Small additional suggestion, say you want to label your line in the legend so that it has some meaning, or take advantage of some of the easy to use options in plot, then using "hold", the ylim from the current axis and the "repmat" is very useful. You can also make multiple vertical lines with some spacing using this technique.
figure
% make some sort of illustration
T = 1000;
A = 0.7;
h = [];
Y = cumsum(sqrt(0.05).*randn(T,1));
X = (1:T)./T;
I = find(X>A);
Y(I) = Y(I(1));
h(1) = plot(X,Y,'-k','linewidth',2);
hold on
dims = get(gca,'ylim');
yy = linspace(dims(1),dims(2),100);
xx = repmat(A,1,100);
h(2) = plot(xx,yy,':r','linewidth',2);
dims = get(gca,'xlim');
xx = linspace(dims(1),dims(2).*A,100);
yy = repmat(Y(I(1)),1,100);
h(3) = plot(xx,yy,':b','linewidth',2);
grid on
G = legend(h,'Particle Motion','Stopping Point','Stopped Value');
set(G,'location','best','interpreter','latex');
Just a thought.

Adrian Peters on 8 Feb 2020
Sorry, but what does (-2:5).^2-1 do? I dont know, how to calculate the ^2-1.
Adrian Peters on 8 Feb 2020
Now it makes sense to me! Thank you a lot!