Scatterplot matlab

Question

I have some problems with a scatter plot.

I am plotting a matrix containing grades per assignment for students e.g. [assignments x grades], but if more than one student gets the same grade in the same assignment, the points will be on top of each other. I want to add a small random number (between -0.1 and 0.1) to the x- and y-coordinates of each dot.

On the x-axis it should be number of assignments and on the y-axis it should be all the grades.

the grades matrix is defined as a 12x4 matrix

My code looks like this:

n_assignments = size(grades,2);  % Total number of assignments.
n_students = size(grades,1);    % Total number of student.
hold on;                    % Retain current plot when adding new plots.
for i = 1:n_assignments     % Loop through every assignment.
% Scatter plot of assignment vs grades for that assignment.
% One assignment on every iteration.
scatter(i*ones(1, n_students), grades(i, :), 'jitter', 'on', 'jitterAmount', 0.1);
end
hold off;                   % Set the hold state to off.
set(gca, 'XTick', 1:n_assignments); % Display only integer values in x-axis.
xlabel('assignment');       % Label for x-axis.
ylabel('grades');           % Label for y-axis.
grid on;                    % Display grid lines.

But I keep getting the error message:

X and Y must be vectors of the same length.

Answer 1

Please note that the scatter plot jitter is an undocumented feature. You can also have semi-transparent markers in line and scatter plots, which could be another alternative to solve your current problem.

I will cover the scatter 'jitter' feature in this answer.

Note that 'jitter' only affects the x-axis but not the y-axis (more info on Undocumented Matlab).

Have a look at this example I made based on your description:

Suppose you have a class with 20 students and they have completed 5 assignments. The grades for the assignments are stored in a matrix (grades) where the rows are the assignments and the columns are the students.

Then I simply generate a scatter plot of the data in the grades matrix, one row at a time, in a for loop and using hold on to keep all the graphics on the same figure.

n_assignments = 5;  % Total number of assignments.
n_students = 20;    % Total number of students.

grades = randi(10, n_assignments, n_students);  % Random matrix of grades.

hold on;                    % Retain current plot when adding new plots.
for i = 1:n_assignments     % Loop through every assignment.
    % Scatter plot of assignment vs grades for that assignment.
    % One assignment on every iteration.
    scatter(i*ones(1, n_students), grades(i, :), 'jitter', 'on', 'jitterAmount', 0.1);
end
hold off;                   % Set the hold state to off.

set(gca, 'XTick', 1:n_assignments); % Display only integer values in x-axis.
xlabel('assignment');       % Label for x-axis.
ylabel('grades');           % Label for y-axis.
grid on;                    % Display grid lines.

This is the result:

---

If you still want to add jitter in the y-axis, you would have to do that manually by adding random noise to your grades data, which is something I personally wouldn't recommend, because the grades in the scatter plot could get mixed, thus rendering the plot completely unreliable.