MATLAB Answers

Philip
6

Adding vertical line to plot?

Asked by Philip
on 25 Feb 2011
Latest activity Answered by Julian Williams on 9 Feb 2019
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!

  1 Comment

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])

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10 Answers

Answer by Michelle Hirsch on 29 Jan 2016
 Accepted Answer

Another simple option that works quite well is hline and vline from the File Exchange:

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Answer by Muhammad
on 8 Jul 2014

line([x x], [y1 y2]); is the easy command;

  3 Comments

you are a genius!

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Answer by Mark
on 12 Mar 2013
Edited by 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).

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Answer by carolina franco on 26 Oct 2017
Edited by 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:

  2 Comments

This one works fantastically...Thanks a lot :D cheers!!
This is AMAZING!!! because it makes the line automatically covering the data area of the plot. So I do not need to do extra work of finding where the line should start and should end. It works for several different plots that had diferent y-axis ranges.

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Answer by 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])

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Answer by James
on 28 Mar 2014
Edited by 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');

  2 Comments

Why is there no documentation on this function? It works great but it is difficult to motivate putting undocumented methods in code that I share with others.
@Steven That's because undocumented features can be removed at any time, as this feature was.

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Answer by Jos (10584)
on 8 Jul 2014

You might also be interested in GRIDXY on the File Exchange:

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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.

  2 Comments

If you use line() instead of plot() then you do not need the "hold". line() is one of the primitives that always adds to the current plot; it is the "high level plotting routines" that clear the current axes before plotting and need the "hold"

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Answer by 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.

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Answer by 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.

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