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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))

SP=1; %your point goes here

line([SP SP],get(hax,'YLim'),'Color',[1 0 0])

Michelle Hirsch
on 29 Jan 2016

Edited: Michelle Hirsch
on 2 Apr 2020

Woohoo - this is built into MATLAB now, as of R2018b! You can use xline and yline to create lines with constant x or y values respectively.

Basic usage couldn't be much easier:

If you are on older releases, another option is hline and vline from the File Exchange: http://www.mathworks.com/matlabcentral/fileexchange/1039-hline-and-vline

Walter Roberson
on 21 Feb 2021 at 16:50

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:

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).

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.

Pedro Luis Camuñas García-Miguel
on 13 Apr 2018

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.

I hope this was useful to whoever consults this page.

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.

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