这个 plot() 函数示例在 Octave\\MatLab 中究竟是如何工作的？

Question

I am pretty new in Octave\\MatLab and following a tutorial I am finding some difficulties trying to understand how exactly works this plot() function version.

So I have the following situation:

I have a data file like this:

34.62365962451697,78.0246928153624,0
30.28671076822607,43.89499752400101,0
35.84740876993872,72.90219802708364,0
60.18259938620976,86.30855209546826,1
79.0327360507101,75.3443764369103,1
45.08327747668339,56.3163717815305,0

Where a row is related to a specific student.

• The first value is the vote of an exame-1 grade for the student.
• The first value is the vote of an exame-2 grade for the student.
• the third value could be only 0 or 1 (1 means that the student was admitted to university, 0 means that the student was not admitted to the university)

Then I have this code that load and plot these information into a graphical like this:

This is the code that loads the previous data from a txt file:

%% Initialization
clear ; close all; clc

%% Load Data
%  The first two columns contains the exam scores and the third column
%  contains the label.

data = load('ex2data1.txt');
X = data(:, [1, 2]); y = data(:, 3);

%% ==================== Part 1: Plotting ====================
%  We start the exercise by first plotting the data to understand the 
%  the problem we are working with.

fprintf(['Plotting data with + indicating (y = 1) examples and o ' ...
         'indicating (y = 0) examples.\n']);

plotData(X, y);

% Put some labels 
hold on;
% Labels and Legend
xlabel('Exam 1 score')
ylabel('Exam 2 score')

% Specified in plot order
legend('Admitted', 'Not admitted')
hold off;

At the end call the plotData(X, y); function definied into another file passing it X (the matrix of the students grades for exam-1 and exam-2** and y (the vector that says if this studen passed or not the university selection):

function plotData(X, y)
%PLOTDATA Plots the data points X and y into a new figure 
%   PLOTDATA(x,y) plots the data points with + for the positive examples
%   and o for the negative examples. X is assumed to be a Mx2 matrix.

% Create New Figure
figure; hold on;

% ====================== YOUR CODE HERE ======================
% Instructions: Plot the positive and negative examples on a
%               2D plot, using the option 'k+' for the positive
%               examples and 'ko' for the negative examples.
%

% Find Indices of Positive and Negative Examples
pos = find(y==1); 
neg = find(y == 0);

% Plot Examples
plot(X(pos, 1), X(pos, 2), 'k+','LineWidth', 2, ...
'MarkerSize', 7);
plot(X(neg, 1), X(neg, 2), 'ko', 'MarkerFaceColor', 'y', ...
'MarkerSize', 7);

It is pretty clear for me what it does the only things that I am not understanding are the last 2 lines that plots the examples:

plot(X(pos, 1), X(pos, 2), 'k+','LineWidth', 2, ...
'MarkerSize', 7);
plot(X(neg, 1), X(neg, 2), 'ko', 'MarkerFaceColor', 'y', ...
'MarkerSize', 7);

My doubts are:

1. What exatly represents the 'k+' and ko parameters passed to this plot() function? I think that k+ rendere the + symbol and the ko render the circle symbol in the previous graph. But I am absolutly not sure of this assertion because here I can't find information about this: https://octave.org/doc/v4.0.0/Two_002dDimensional-Plots.html

2. Why in the positive case it is using the parameter LineWidth while in the negative case I am using the MarkerFaceColor ?

3. Why in each of these 2 plot line wrap up after the ... characters?

   plot(X(pos, 1), X(pos, 2), 'k+','LineWidth', 2, ... 'MarkerSize', 7);

What these ... exactly means? I tried to put all on a single line like this:

plot(X(pos, 1), X(pos, 2), 'k+','LineWidth', 2, ... 'MarkerSize', 7);

but doing in this way Octave is brocken and the execution is stoped. Why? What am I missing?

Answer 1

1. The 'k+' and 'ko' are suggesting to plot a black + and a black o. The k is the color and the + and o are the plot types. See https://www.mathworks.com/help/matlab/ref/linespec.html for more detail about the specifications (should be the same for Octave)

2. The 'LineWidth' overwrites the default width of the line for the plus symbol, while the "MarkerFaceColor" overwrites the default fill color of markers that can be filled (eg circles, squares, etc.). Since the LineWidth is not specified for the circle symbols, it just uses the default width. Since the plus symbols are not fillable, the MarkerFaceColor is not necessary.

3. The '...' simply means that the line overflows into the next line. It is similar to \\ or & in some other programming languages.