MATLAB - Add Legend to Axes

MATLAB - Creating Informative Legends for Plots

A legend is a crucial component of any scientific or data-driven plot. It acts as a key, explaining what each line style, marker, or color represents in your visualization. A plot without a legend can be ambiguous or misleading. MATLAB provides the flexible legend function to create clear and descriptive legends.

The modern and most robust method for creating legends is to assign a DisplayName to each plot object as you create it. This decouples the plotting code from the legend creation, leading to cleaner and more maintainable code.

Syntax

% Modern Recommended Method
plot(x1, y1, 'DisplayName', 'Data Series 1')
hold on
plot(x2, y2, 'DisplayName', 'Data Series 2')
hold off
legend

% Legacy / Manual Method
legend('Label 1', 'Label 2', ..., 'Label N')

% Advanced Customization
legend(..., 'Name', Value) % e.g., 'Location', 'northwest'

Syntax Explained

• plot(..., 'DisplayName', name): This name-value pair within your plotting function (like plot, scatter, bar, etc.) assigns a label to that specific graphical object. This is the preferred best practice.
• legend: When called with no arguments, legend automatically finds all plot objects that have a DisplayName property set and creates a legend from them. It intelligently ignores plots without a DisplayName.
• legend('Label1', 'Label2', ...): This manually assigns labels to the plots in the order they were created. This can be brittle; if you change the plotting order, you must also change the legend order.
• legend(..., 'Name', Value): Customizes the legend’s properties. The most common is 'Location' to control its position, but you can also change font size, orientation, and more.

Example 1: The Modern DisplayName Method

Let’s plot sin(x) and cos(x) and use the DisplayName property to create a robust legend.

% 1. Create data
x = linspace(0, 2*pi, 100);

% 2. Create a figure and axes
figure;
ax = gca; % Get current axes handle

% 3. Plot the data, assigning DisplayName to each
plot(ax, x, sin(x), 'b-', 'LineWidth', 2, 'DisplayName', 'sin(x)');
hold on; % Keep the current plot active
plot(ax, x, cos(x), 'r--', 'LineWidth', 2, 'DisplayName', 'cos(x)');
hold off; % Release the plot

% 4. Add titles and labels for context
title(ax, 'Sine and Cosine Functions');
xlabel(ax, 'Angle [rad]');
ylabel(ax, 'Value');
grid on;

% 5. Create the legend automatically
legend;

Benefit: If you decide to add another plot, say tan(x), you just add another plot command with its own DisplayName. The legend command doesn’t need to change at all.

Example 2: Customizing Legend Location and Appearance

Sometimes the default legend location covers up important data. You can easily move it.

x = linspace(0, 10, 100);
y1 = exp(-0.2*x) .* sin(x);
y2 = exp(-0.1*x) .* cos(x);

figure;
plot(x, y1, 'DisplayName', 'Damped Sine');
hold on;
plot(x, y2, 'DisplayName', 'Damped Cosine');
hold off;

title('Damped Oscillations');
xlabel('Time [s]');
ylabel('Amplitude');
grid on;

% Create the legend and specify properties
lgd = legend('Location', 'northeast', 'FontSize', 12);
title(lgd, 'Signals'); % Add a title to the legend itself!

Pro Tip: Common 'Location' values include 'best', 'northwest', 'southwest', 'northeast', 'southeast', and 'center'. Using 'best' lets MATLAB try to find the location with the least data overlap.

Example 3: Creating a Legend for a Subset of Plots

What if you have helper plots (like a trend line) that you don’t want to include in the legend? The DisplayName method handles this automatically. Alternatively, you can manually specify which plots to include.

x = linspace(0, pi, 50);

y1 = cos(x);
p1 = plot(x, y1, 'DisplayName', 'cos(x)'); % Plot with DisplayName

hold on

y2 = 0.5*cos(2*x);
plot(x, y2); % Plot WITHOUT DisplayName

y3 = 0.25*cos(3*x);
p3 = plot(x, y3, 'DisplayName', '0.25*cos(3x)'); % Plot with DisplayName

hold off
title('Including only relevant data in legend');

% Method 1: Automatic (Recommended)
% Simply calling legend will only include p1 and p3
legend('Location', 'southwest');

% --- OR ---

% Method 2: Manual Control (if you need to override)
% legend([p1, p3]); % This achieves the same result by passing handles

In this example, the second plot of 0.5*cos(2*x) is treated as a visual aid and is correctly excluded from the legend because it lacks a DisplayName.

Common Issues and Debugging

• Warning: Ignoring extra legend entries: This happens if you use the manual method legend('A', 'B', 'C') but only have two plots on your axes. Ensure the number of labels matches the number of plotted series.
• Legend is Empty: This can happen if you call legend when no plots have a DisplayName set. Double-check that you’ve added the 'DisplayName' property to your plotting commands.
• Programmatic Control: For highly dynamic plots, you can programmatically turn off an item’s inclusion in the legend by setting its HandleVisibility property: p.HandleVisibility = 'off';. This is an advanced technique for complex GUIs or apps.