Draw arrow in a plot

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francisco gonzalez
francisco gonzalez on 22 May 2015
Hi all, i have a question. I am creating a GUI that import real time data. I want draw a vertical line or a arrow if a condition is met.
For example:
n=length(p);
for i=1:n;
if p>100 && t>100
annotation=('arrow');
end
end
Is this correct for draw arrow in a plot in real time?
Thank you very much

Accepted Answer

Geoff Hayes
Geoff Hayes on 23 May 2015
Francisco - you may want to read the documentation on annotation. Your code of
annotation=('arrow');
would not work since you are assigning the string arrow to a variable called annotation. Instead you would want to do something like
X = [0.5 0.5];
Y = [0 0.5];
annotation('arrow',X,Y);
which would draw an arrow starting at the coordinate (0.5,0) and ending at (0.5,0.5). (Note that the coordinates are specified in normalized figure units which means that the arrow would be drawn in the middle.)
While you need not provide the X and Y inputs to this function, be aware that the default values will be used in place (of them) so the arrow may not be drawn where you expect it to be.
  6 Comments
Geoff Hayes
Geoff Hayes on 23 May 2015
Are there lower and upper bounds for your Variable array? I realize that the example indicates that the values will be between -6 and +6 but that you want to draw an arrow at some point that is greater than six.
If we assume that the minimum value for y is -6 and that the maximum is 100, then we could do the following
% create a figure and set the axes limits
figure;
minX = 1; maxX = 59;
xlim([minX maxX]);
minY = 0; maxY = 6; upperY = 100;
ylim([minY upperY]);
Now we have to handle the incoming data in Variable or (in my example) varData.
% create some random data that is in the interval [-6, 6].
varData = minY + rand(1,maxX)*(maxY - minY);
% set two of these values to be greater than 6
varData(23) = 19;
varData(42) = 91;
% find those values in varData that are greater than 6
dataOutsideOfInterval = varData > maxY;
if any(dataOutsideOfInterval)
xIdcs = find(dataOutsideOfInterval==1);
for k=1:length(xIdcs)
x = xIdcs(k)/maxX;
y = (varData(xIdcs(k))-minY)/(upperY-minY);
annotation('arrow',[x x], [(maxY-minY)/(upperY-minY) y]);
end
end
The above doesn't quite work the way I would like as it seems to take into account the complete figure and not the axes (so the arrows aren't exactly starting and pointing where I would like). But you should be able to start with this and find a good solution.
francisco gonzalez
francisco gonzalez on 23 May 2015
Thank you very much Geoff. I am going to study this code and apply to my situation.

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