bar plot - add multiple y values for same x

Hello,

I have an array A which contains some integer values, an array B which contains indices:

A = [ 1 2 3 4 5 6]

B = [3 1 2 1 2 1]

let's say that the maximal value at B can be 5 and some indices can also be blank.

I'd like to make a bar plot which will contain multiple bars where it's possible more than one value for the index:

How can it be done?

Thanks,

Avihai

Answer 1

[BhaTTa]

Hey @avihai, please refer to the below example code , please take it as a reference and modify it accordingly

A = [1 2 3 4 5 6];
B = [3 1 2 1 2 1];

% Define the maximum possible index value from B.
% If B could contain indices larger than 5, you'd use max(B) instead of a fixed 5.
max_b_value = 5; 

% 1. Group the A values by their corresponding B indices
% We'll use a cell array to store values for each index, as they can have different counts.
grouped_data = cell(1, max_b_value);

for i = 1:length(A)
    current_index = B(i);
    current_value = A(i);
    
   
    if current_index >= 1 && current_index <= max_b_value
        grouped_data{current_index} = [grouped_data{current_index}, current_value];
    else
        warning('Index %d from B is out of the expected range (1 to %d). Skipping.', current_index, max_b_value);
    end
end

% 2. Prepare the data for the bar plot
% The 'bar' function expects a matrix where each column represents an X-axis group
% and rows represent the different bars within that group.
% We need to pad shorter groups with NaN (Not a Number) so all columns have the same length.

% Find the maximum number of values for any single index
max_bars_per_index = 0;
for i = 1:max_b_value
    max_bars_per_index = max(max_bars_per_index, length(grouped_data{i}));
end


plot_matrix = NaN(max_bars_per_index, max_b_value);

for i = 1:max_b_value
    current_group_values = grouped_data{i};
    if ~isempty(current_group_values)
        plot_matrix(1:length(current_group_values), i) = current_group_values;
    end
end

figure;

% Use the 'bar' function. By default, it creates grouped bars if the input is a matrix.
h = bar(plot_matrix); 


xticks(1:max_b_value); 
xticklabels(arrayfun(@num2str, 1:max_b_value, 'UniformOutput', false)); % Label ticks with numbers

xlabel('Index from B');
ylabel('Value from A');
title('Values from A Grouped by Index from B');
grid on;