String Annotations for Plots

Did you know that you can customize the direction of text in MATLAB plots? There are times when such annotations work better than if they were purely horizontal or vertical.  

Contents

Sunspot Data

Let's look at some sunspot data. First I'll load it into MATLAB.

s = load('sunspot.dat');

Separate the year from the actual number of sunspots.

year = s(:,1);
spots = s(:,2);

Show the data in a bar plot.

bar(year,spots)

Annotate the Year with the Maximum Number of Sunspots

[smax, indmax] = max(spots);

The historical record shows the year

yearmax = year(indmax)

yearmax =
        1957

with the maximum number of sunspots, 190.

hold on
plot(yearmax, smax, 'm*')
hold off
ht = text(yearmax, smax,['  ',int2str(round(smax)), ' sunspots  ']);

Use Handle Graphics to Alter the Annotation Orientation

Let's try labeling the maximum so the label is right-justified now.

set(ht,'HorizontalAlignment','right')

Now change the label so it is at an angle, something I might want if I were planning to label several more points on the graph.

set(ht,'Rotation',45)

Note Other Maxima

The sunspot cycle has a periodicity of about 11 years. Let's see which other years have a high number of sunspots.

[maxssa, indssa] = sort(spots,'descend');
year(indssa(1:10))

ans =
        1957
        1958
        1959
        1979
        1980
        1778
        1947
        1956
        1981
        1870

We see some years near the maximum year, 1957, then another cluster near 1979, and the next at 1778. Let me grab the first five "distinct" clusters.

idxdistinct = [1 4 6 7 8];
year(indssa(idxdistinct))

ans =
        1957
        1979
        1778
        1947
        1956

Label Extra Peaks

Let's label these few other peaks in a similar manner to the first one.

nearMaxYears = year(indssa(idxdistinct(2:end)))
ssaMax = spots(indssa(idxdistinct(2:end)))
hold on
plot(nearMaxYears, ssaMax, 'mo')
hold off
hn = text(nearMaxYears, ssaMax,['  high sunspot activity  '],...
    'HorizontalAlignment','Right','Rotation',25);
set(ht,'Rotation',0)

nearMaxYears =
        1979
        1778
        1947
        1956
ssaMax =
  155.4000
  154.4000
  151.6000
  141.7000

Write Around a Circle

Just for fun now, let's write a phrase around a circle of radius 1.

clf, box on
phrase = 'ring around the collar ';
num = length(phrase)
angles = 0:(360/num):359;
x = cos(angles*pi/180);
y = sin(angles*pi/180);
for ind = 1:length(phrase)
    text(x(ind),y(ind),phrase(ind),'Rotation',angles(ind),...
        'HorizontalAlignment','center')
end
axis equal, axis([-1.1 1.1 -1.1 1.1])

num =
    23

References

There is a whole lot more you can do with text annotation in MATLAB. Here are some references for the language aspects of working with text. Though I don't want to turn this blog into one chiefly about graphics, I think I can cover some aspects here and still talk about it in terms of the language. Any suggestions? Please post them here.

Get the MATLAB code

Published with MATLAB® 7.4