Specify annotation position with respect to x- and y-axes values?

Dear all,

Is there a way the define the starting and end point of an annotation object such as an arrow in terms of the data being plotted and not as either "normalized" coordinates (x- and y-positions between 0 and 1) or a physical length measurement such as cm?

E.g. I am plotting a time from 0 to 180 s on the x-axis and a concentration between 0 and 0.1 concentration units on the y-axis. Now I would like to draw an arrow starting at 90 s, 0.04 concentration units and going to 120 s, 0.09 concentration units.

I have already considered to simply "normalize" those positions myself (e.g. 90 s/180 s = 0.5 --> x-position in normalized coordinates). This does, however, not work, since 0 and 1 refer to points in the whole figure (the window in which the plot appears) and not to the area of the plot itself.

Thanks for any input! Clemens

[Clemens]

Here's a quick illustration of what I mean. I simply drew the arrow by hand. If I get Matlab to show me the code for it, it gives

annotation(figure1,'arrow',[0.354235423542354 0.224422442244224],...
  [0.526863777089783 0.390092879256966]);

but the end point's x-coordinate (0.2244...) does not correspond to the 20 s (20 s / 180 s = 0.1111...) where 180 s is the "length" of my x-axis in the units that I want to plot. 

Answer 1

[Star Strider]

You need to access the 'Position' property of the figure object you’re using. It gives the left lower corner coordinates and the width and height values. With a few lines of code, you can normalise your arrow object positions with respect to them. See the documentation on Axes Properties for details.

Then, experiment! This will keep you intensely occupied for a few minutes!

[Clemens]

Thanks heaps, I got it! For everyone, here's a short instruction: Looking up, as suggested by Star Strider, the 'Position' Property in the Axes Properties documentation, I found:

The third element in the vector, 0.7750, corresponds to the default width of the plot within the figure in normalized coordinates (i.e. the figure / the window containing the plot has a width of 1 length units). I then calculated the x-position of my 20s-marker in normalized coordinates as 20/(180*0.7750) (first normalizing to the length of the x-axis, then to the size of the plot with respect to the window. Haven't tried it on this example but on another one, but I assume it works.

[Star Strider]

As always, my pleasure!

[DrBones]

Edit: I am terribly sorry, both for writing things I could not make sense of a couple months later and for not replying sooner Star Strider. Apparently, I did not get notified.

This comment is replaced with a way too long new comment which hopefully makes up for it since it also scales to negative x and hopefully isn't totally out of place here. My apologies in advance if it is.

    % define domain and function
    x = linspace(-10, 10, 100);
    sigma = 3;
    gaussian = exp(-x.^2/(2*sigma^2));

    % plot the function and some labels
    figure
    plot(x, gaussian)
    hold on
    xlabel('x')
    ylabel('y')

    HM = 0.5 % Half of maximum
    HWHM = 3.53 % Half width at half maximum
    pos = get(gca, 'Position');

    % Positins for the end of the Arrow in data units.
    xPosition = 2;
    yPosition = 0.3;

    % Create a textarrow annotation at the coordinates in data units
    % the textarrow coordinates are given [end_x, head_x], [end_y, head_y]
    ta1 = annotation('textarrow',...
        [(xPosition + abs(min(xlim)))/diff(xlim) * pos(3) + pos(1),...
         (HWHM + abs(min(xlim)))/diff(xlim) * pos(3) + pos(1) ],... 
        [(yPosition - min(ylim))/diff(ylim) * pos(4) + pos(2),...
         (HM - min(ylim))/diff(ylim) * pos(4) + pos(2)]);

    % The data units are rescaled to normalized units for x via
    % x_norm = ( x_data + abs(min(xlim)))/diff(xlim) * pos(3) + pos(1)
    % and for y via ( does NOT work for negative values (see x for this)
    % y_norm = (y_data - min(ylim))/diff(ylim) * pos(4) + pos(2)

    % Set some text
    ta1.String = ['HWHM: ', num2str(HWHM, 3), ];

    % Also for negative number now :). "line" accepts the same style
    % coordinates
    Start = [-8, 0.5]; % x, y
    End   = [4, 0.1];  % x , y
    ta2   = annotation('line',...
        [(End(1) + abs(min(xlim)))/diff(xlim) * pos(3) + pos(1),...
         (Start(1) + abs(min(xlim)))/diff(xlim) * pos(3) + pos(1) ],... 
        [(End(2) - min(ylim))/diff(ylim) * pos(4) + pos(2),...
         (Start(2) - min(ylim))/diff(ylim) * pos(4) + pos(2)]);

This results in

Thanks again for all the help!

Also unrelated —

My orthopedic knowledge is, apart from my namesake, unfortunately rather lacking. I am more involved in the imaging side :)

[Star Strider]

More information would be helpful, as would more illustrative example code, since we don’t know what you’re doing, and providing a context is always appropriate. I assume ‘x_data’ is your independent variable vector. Also, is this robust to ‘x_data’ having negative values?

Unrelated —

‘DrBones’ — Ortho? B/C IM here!

[Star Strider]

In the inteiriim (early 2023), I came up with my own solution —

    % define domain and function
    x = linspace(-10, 10, 100);
    sigma = 3;
    gaussian = exp(-x.^2/(2*sigma^2));

    % plot the function and some labels
    figure
    plot(x, gaussian)
    hold on
    xlabel('x')
    ylabel('y')

    HM = 0.5 % Half of maximum

HM = 0.5000

    HWHM = 3.53 % Half width at half maximum

HWHM = 3.5300

    pos = get(gca, 'Position');

    % Positins for the end of the Arrow in data units.
    xPosition = 2;
    yPosition = 0.3;

    Start = [-8, 0.5]; % x, y
    End   = [4, 0.1];  % x , y

    xapf = @(x,pos,xl) pos(3)*(x-min(xl))/diff(xl)+pos(1);                % 'x' Annotation Position Function
    yapf = @(y,pos,yl) pos(4)*(y-min(yl))/diff(yl)+pos(2);                % 'y' Annotation Position Function
    xl = xlim;
    yl = ylim;
    pos = gca().Position;
    
    annotation('line', xapf([Start(1) End(1)],pos,xl), yapf([Start(2) End(2)],pos,yl))
    annotation('textarrow', xapf([xPosition HWHM],pos,xl), yapf([yPosition HM],pos,yl), String=sprintf('HWHM = %.2f',HWHM))

.

Answer 2

[Jorg Woehl]

This is an older post, but the following is perhaps useful to others:

If horizontal arrows are ok, data units can be used by associating the annotation with the current axes:

hf = figure;
x = 0:180; f = @(x) 0.09*exp(-x/18);
plot(x, f(x));

ha = annotation('arrow');
ha.Parent = hf.CurrentAxes;  % associate annotation with current axes
% now you can use data units
ha.X = [50 20];
ha.Y = [f(20) f(20)];

This also works for other annotation types and even if they don't have a horizontal orientation. (Arrows and doublearrows are an exception because the arrowheads will not align properly.)

Otherwise, there is also the excellent coord2norm function on File Exchange that converts data units to normalized units.

[Jonas Maturo]

Great info!

[Mateusz Tymiakin]

thanks

[Eitvydas Karauskas]

Hey guys, I really need coord2norm function for my code, since I need to draw some arrows on my map. However, after doing everything as instructed it doesn't work. It still says that "X and Y values must be between 0 and 1". I'm including part of my code.

Any help would be greatly appreciated ;)

%defining latitude and longitude
latlim=[53.8 55.5];
lonlim=[24 26];
%loading world map
map=worldmap(latlim,lonlim);
%loading lithuania borders from external source and displaying it as a
%geografic coordinates
country=shaperead('gadm36_LTU_0.shp','usegeocoords',true,'BoundingBox', [lonlim', latlim']);
geoshow(map,country,'facecolor',[1 1 1],'linewidth',2);

...
    
SOGBIlon = 25.195833;
SOGBIlat = 54.213056;
geoshow( SOGBIlat, SOGBIlon, 'marker','^', 'markerfacecolor','[0 0.45 0.74]','markeredgecolor','[0 0.45 0.74]','markersize',6);
textm(SOGBIlat,SOGBIlon, 'SOGBI','fontweight','bold','color','black','fontsize',6);

NOLNAlon = 25.098472;
NOLNAlat = 54.362944;
geoshow( NOLNAlat, NOLNAlon, 'marker','^', 'markerfacecolor','[0 0.45 0.74]','markeredgecolor','[0 0.45 0.74]','markersize',6);
textm(NOLNAlat,NOLNAlon, 'NOLNA','fontweight','bold','color','black','fontsize',6);

[normx, normy] = coord2norm (map,[SOGBIlon NOLNAlon], [SOGBIlat NOLNAlat]);
annotation ('arrow', normy, normx);

[Brent F]

Excellent answer - much better than doing the conversion ourselves each time.

Preferably, we'd just specify the arrowhead x,y + label text.

function [] = LabelArrow (fig, x, y, labelStr)
    a = annotation('textarrow');
    a.Parent = fig.CurrentAxes;
    a.X = [x x]; a.X = .90*a.X; a.X(1) = .8*a.X(1);
    a.Y = [y y];
    a.String = labelStr;
    a.LineWidth = 1;
end

But the method of chosing the starting point and direction the arrow points above is a hack and doesn't always look right.

Is there any tool/plugin that handles figuring out where the text should go in order to avoid overwriting the graph and drawing the arrow from there?

[Marc Compere]

coord2norm() is super helpful

[Marc Compere]

@Eitvydas Karauskas, if you get the error "X and Y values must be between 0 and 1", it's from Matlab, not coord2norm(). It means the axes you've got are not large enoguh to place the new annotation using coordinates from coord2norm(). Try increasing your axes limits then try the annotation again.

[Jiarui Kang]

coord2norm is nice and intuitive. Thanks for sharing.

[Trevor]

Excellent addition, thanks so much!

[Swaroop]

Thank you very much!

[Francisco Diaz]

Thank you very much

Answer 3

[DGM]

As a spin off @Jorg Woehl's suggestion, I'll offer my own.

I think it's worth pointing out that these solutions tend to be version-dependent -- at least dependent on whether we're post-R2014b or not. Obviously, we can't manipulate properties of annotation objects using dot notation in legacy versions, but even using get/set to move the annotation tends to result in it being drawn in unexpected ways.

But precalculating the position and specifying it when creating the annotation works fine from R2009b to R2019b:

clf
x = 0:180; f = @(x) 0.09*exp(-x/18);
plot(x, f(x));
[xx yy] = datc2figc([50 20],[f(20) f(20)]);
annotation('arrow',xx,yy);

I used the attached file for coordinate conversion, but I doubt it's much different than coord2norm().

[Marc Compere]

hello, agreed. this datc2figc.m script looks simpler than coord2norm.m. coord2norm handles the two cases you mentioned, pre and post 2019, and I liked it but wasn't able to commit to it because it failed to handle reversed Y-axis coordinates.

There's definitely room for simplification and improvements. If you post this as your own Mathworks File Exchange entry it can help the community.

[Marc Compere]

it's bizarre that data coordinates are not built into Matlab directly. Its the single most straightforward use for an arrow annotation.

[DGM]

datc2figc() was really just an ad-hoc thing I threw together in frustration. I'm sure there's better stuff out there.