Octave – Basics of Plotting Data

Octave has some in-built functions for visualizing the data. Few simple plots can give us a better way to understand our data. Whenever we perform a learning algorithm on an Octave environment, we can get a better sense of that algorithm and analyze it. Octave has lots of simple tools that we can use for a better understanding of our algorithm. 
In this tutorial, we are going to learn how to plot data for better visualization and understanding it in the Octave environment.
Example 1 : Plotting a sine wave using the plot() and and sin() function: 

MATLAB

% var_x for the y-axis
var_x = [0:0.01:1];

% var_y for the y-axis
var_y = sin(4 * pi * var_x);

% plotting the graph
plot(var_x, var_y);

Output :

Example 2 : Plotting a cosine wave using the plot() and and cos() function: 

MATLAB

% var_x for the y-axis
var_x = [0:0.01:1];

% var_y for the y-axis
var_y = cos(3 * pi * var_x);

% plotting the graph
plot(var_x, var_y);

Output :

Example 3 : We can plot, one plot over another plot by holding the previous plot with the hold on command. 

MATLAB

% declaring variable var_x
var_x = [0:0.01:1];

% declaring variable var_y1
var_y1 = sin(4 * pi * var_x);

% declaring variable var_y2
var_y2 = cos(3 * pi * var_x);

% plot var_x with var_y1
plot(var_x, var_y1);

% hold the above plot or figure
hold on;

% plot var with var_y2 with red color
plot(var_x, var_y2, 'r');

Output :

Example 4 : We can add labels for the x-axis and the y-axis along with the legends and title with the below code. 

MATLAB

% declaring variable var_x
var_x = [0:0.01:1];

% declaring variable var_y1
var_y1 = sin(4 * pi * var_x);


% declaring variable var_y2
var_y2 = cos(3 * pi * var_x);

% plot var_x with var_y1
plot(var_x, var_y1);

% hold the above plot or figure
hold on;

% plot var with var_y2 with red color
plot(var_x, var_y2, 'r');

% adding label to the x-axis
xlabel('time');

% adding label to the y-axis
ylabel('value');

% adding title for the plot
title('my first plot');

% add legends for these 2 curves
legend('sin', 'cos');

Output :

Example 5 : We can also plot data on different figures. 

MATLAB

% declaring variable var_x
var_x = [0:0.01:1];

% declaring variable var_y1
var_y1 = sin(4 * pi * var_x);

% declaring variable var_y2
var_y2 = cos(3 * pi * var_x);

% plot var_x and var_y1 on figure 1
figure(1);
plot(var_x,var_y);

% plot var_x and var_y2 on figure 2
figure(2);
plot(var_x,var_y2);

Output :

Example 6 : We can divide a figure into a m x n grid using the subplot() function. In the below code the first 2 parameter shows m and n and 3rd parameter is the grid count from top to left. 

MATLAB

% var_x for the y-axis
var_x = [0:0.01:1];

% var_y for the y-axis
var_y = sin(4 * pi * var_x);

% plot the var_x and var_y on a 3x3 grid
% at 4 position counting from top to left
subplot(3, 3, 4), plot(var_x, var_y);

Output :

Example 7 : We can change the axis values of any plot using the axis() function.

MATLAB

% declaring variable var_x
var_x = [0:0.01:1];

% declaring variable var_y1
var_y1 = sin(4 * pi * var_x);

% declaring variable var_y2
var_y2 = cos(3 * pi * var_x);

% plot var_x with var_y1
plot(var_x, var_y1);

% hold the above plot or figure
hold on;

% plot var with var_y2 with red color
plot(var_x, var_y2, 'r');

% adding label to the x-axis
xlabel('time');

% adding label to the y-axis
ylabel('value');

% adding title for the plot
title('my first plot');

% add legends for these 2 curves
legend('sin', 'cos');

% first 2 parameter sets the x-axis
% and next 2 will set the y-axis
axis([0.5 1 -1 1])

Here the first 2 parameters shows the range of the x-axis and the next 2 parameters shows the range of the y-axis. 
Output :

Example 8 : We can save our plots in our present working directory : 

MATLAB

print -dpng 'plot.png'

In order to print this plot at our desired location, we can use cd with it as shown below : 

MATLAB

cd '/home/dikshant/Documents'; print -dpng 'plot.png'

We can close a figure/plot using the close command.
Example 9 : We can visualize a matrix using the imagesc() function. 

MATLAB

% creating a 10x10 magic matrix
matrix = magic(10)

% plot the matrix
imagesc(matrix)

Output :

matrix =

    92    99     1     8    15    67    74    51    58    40
    98    80     7    14    16    73    55    57    64    41
     4    81    88    20    22    54    56    63    70    47
    85    87    19    21     3    60    62    69    71    28
    86    93    25     2     9    61    68    75    52    34
    17    24    76    83    90    42    49    26    33    65
    23     5    82    89    91    48    30    32    39    66
    79     6    13    95    97    29    31    38    45    72
    10    12    94    96    78    35    37    44    46    53
    11    18   100    77    84    36    43    50    27    59

The above plot is of 10×10 grid, each grid represents a value with a color. The same color value results in the same color. 
We can also make a color bar with this plot to see which value corresponds to which color using the colorbar command. We can use multiple commands at a time by separating them with a comma(,) in Octave environment. 

MATLAB

% creating a 10x10 magic matrix
matrix = magic(10)

% plot this matrix with showing colorbar on the right of it
imagesc(matrix), colorbar;

Output :

Drawing the magic square with a gray-scale colormap : 

MATLAB

% creating a 10x10 magic matrix
matrix = magic(10)

% plot this matrix with colorbar and gray colormap
imagesc(matrix), colorbar, colormap gray;

Output :