How to change colors in my legend that is classified into three intervals?

Hi!

I have a code that looks like this:

function class_post(lon, lat, var, class)

m = length(var);
hold on 

color = [rgb('blue'); rgb('green'); rgb('red')];
% color = [rgb('red'); rgb('orange'); rgb('yellow'); ...
%          rgb('green'); rgb('cyan'); rgb('blue'); rgb('purple')];
size = [2 4 6 8 9.2 10.5 12];
bins = {};

if isempty(class)
    a = min(var); 
    b = max(var); 
    class = [a + (b - a) / 3: (b - a) / 3: b - (b - a) / 3];
    
    bins = {sprintf(' < %1.3f', class(1))};
    ind = find(var < class(1));
    m_plot(lon(ind), lat(ind), 'o', 'color', color(1,:), 'markerfacecolor', color(1,:));
    for i = 1:length(class)-1 % Loop until the second-to-last element
        c = color(i+1,:); 
        s = size(i+1);
        bins = [bins; sprintf('%1.3f - %1.3f', class(i), class(i+1))]; 
        ind = find(var >= class(i) & var < class(i+1));
        m_plot(lon(ind), lat(ind), 'o', 'color', c, 'markerfacecolor', c);
    end
    ind = find(var >= class(end)); % Use end to access the last element
    m_plot(lon(ind), lat(ind), 'o', 'color', color(end,:), 'markerfacecolor', color(end,:));
    bins = [bins; sprintf(' > %1.3f', class(end))];
else
    for i = 1:length(class)
        c = color(i,:); 
        s = size(i);
        bins = [bins; sprintf('%1.3f - %1.3f', class(i), class(i+1))]; 
        ind = find(var >= class(i) & var < class(i+1));
        m_plot(lon(ind), lat(ind), 'o', 'color', 'k', 'MarkerSize', s, 'LineWidth', 1);
    end
end

% Plot map details
m_gshhs_f('patch', [.8 .8 .8], 'LineWidth', 1.2);
m_grid('tickdir', 'out', 'linewidth', 1.2, 'xtick', 4, 'ytick', 5);
xlabel('Longitude (\circN)', 'fontsize', 12);
ylabel('Latitude (\circE)', 'fontsize', 12);

% For legend
h = legend(bins, 'location', 'EastOutside');
ax1 = gca;  
pos = get(ax1, 'position');

I am trying to plot my data with three classes (<less than(blue), =no change(green) and >greater than(red)). However, when I run this code only the green and red colors shows. How can I fix this code to include all colors in my legend?

Thank you!

[Walter Roberson]

I wonder whether you need

hold on

I notice that you draw four lines, not three:

1. less than the first class -- blue
2. range first class to second class -- green
3. range second class to third class -- red
4. greater than third class -- red

Your legend should reflect these four entries.

[Sofia Quillope]

in which line should I put the "hold on'?

Thank you

[Walter Roberson]

Put the hold on after the first m_plot

[Voss]

@Sofia Quillope: General advice: avoid naming variables the same as built-in functions, e.g., naming your variable size will prevent you from using the built-in function size().

[Dyuman Joshi]

class() is also a built-in function, which OP has used as a variable name.

Answer 1

[Voss]

Try storing the line handles returned from the m_plot calls, and then use those line handles in the legend call. That fixed the problem for me when I tested it.

Something like this (note the new variable hl is the line handle array):

function class_post(lon, lat, var, class)

m = length(var);
hold on 

color = [rgb('blue'); rgb('green'); rgb('red')];
% color = [rgb('red'); rgb('orange'); rgb('yellow'); ...
%          rgb('green'); rgb('cyan'); rgb('blue'); rgb('purple')];
size = [2 4 6 8 9.2 10.5 12];
bins = {};

if isempty(class)
    a = min(var); 
    b = max(var); 
    class = [a + (b - a) / 3: (b - a) / 3: b - (b - a) / 3];
    
    bins = {sprintf(' < %1.3f', class(1))};
    ind = find(var < class(1));
    
    % pre-allocate line handle array:
    hl = gobjects(1,numel(class)+1);
    
    % store the first line in hl(1):
    hl(1) = m_plot(lon(ind), lat(ind), 'o', 'color', color(1,:), 'markerfacecolor', color(1,:));
    
    for i = 1:length(class)-1 % Loop until the second-to-last element
        c = color(i+1,:); 
        s = size(i+1);
        bins = [bins; sprintf('%1.3f - %1.3f', class(i), class(i+1))]; 
        ind = find(var >= class(i) & var < class(i+1));
        
        % store each line:
        hl(i+1) = m_plot(lon(ind), lat(ind), 'o', 'color', c, 'markerfacecolor', c);
        
    end
    ind = find(var >= class(end)); % Use end to access the last element
    
    % store the last line:
    hl(end) = m_plot(lon(ind), lat(ind), 'o', 'color', color(end,:), 'markerfacecolor', color(end,:));
    
    bins = [bins; sprintf(' > %1.3f', class(end))];
else
    for i = 1:length(class)
        c = color(i,:); 
        s = size(i);
        bins = [bins; sprintf('%1.3f - %1.3f', class(i), class(i+1))]; 
        ind = find(var >= class(i) & var < class(i+1));
        m_plot(lon(ind), lat(ind), 'o', 'color', 'k', 'MarkerSize', s, 'LineWidth', 1);
    end
end

% Plot map details
m_gshhs_f('patch', [.8 .8 .8], 'LineWidth', 1.2);
m_grid('tickdir', 'out', 'linewidth', 1.2, 'xtick', 4, 'ytick', 5);
xlabel('Longitude (\circN)', 'fontsize', 12);
ylabel('Latitude (\circE)', 'fontsize', 12);

% For legend
% explicitly tell legend which lines use in the legend:
h = legend(hl, bins, 'location', 'EastOutside');
ax1 = gca;  
pos = get(ax1, 'position');

[Voss]

Unrelated to your question: note that in the else block

for i = 1:length(class)
    c = color(i,:); 
    s = size(i);
    bins = [bins; sprintf('%1.3f - %1.3f', class(i), class(i+1))]; 
    ind = find(var >= class(i) & var < class(i+1));
    m_plot(lon(ind), lat(ind), 'o', 'color', 'k', 'MarkerSize', s, 'LineWidth', 1);
end

you're going to have an error when i is length(class) because you try to access class(i+1).