Effective Legend Placement Strategies for MATLAB Figures

Effective Legend Placement Strategies for MATLAB Figures

Legends are crucial elements in data visualization, providing context and clarity by identifying the different data series within a plot. A well-placed and formatted legend enhances the figure’s readability and allows the audience to quickly grasp the information presented. In MATLAB, while the default legend placement often suffices, fine-tuning its position and appearance can significantly improve the overall effectiveness of your visualizations. This article delves into the intricacies of legend placement in MATLAB, offering a comprehensive guide to optimizing legend position, appearance, and interaction for diverse plotting scenarios.

1. Understanding the Basics of Legend Creation and Placement

The simplest way to create a legend in MATLAB is using the legend function. Providing the plot handles or labels as arguments generates a legend automatically. MATLAB’s default placement algorithm aims to minimize overlap with the plotted data. However, this automatic placement isn’t always ideal, particularly in complex plots.

matlab
x = linspace(0, 2*pi, 100);
y1 = sin(x);
y2 = cos(x);
plot(x, y1, 'r', x, y2, 'b');
legend('sin(x)', 'cos(x)');

2. Manual Legend Placement Options:

MATLAB offers several methods to manually control the legend’s location:

• ‘Location’ Specifier: This is the most common approach. The legend function accepts a ‘Location’ argument followed by a string specifying the desired position. Common options include:

  • 'north', 'south', 'east', 'west': Places the legend outside the plot axes along the specified edge.
  • 'northeast', 'northwest', 'southeast', 'southwest': Places the legend in the corners of the plot axes.
  • 'best': (Default) Attempts to find the best location to minimize overlap with the plotted data.
  • 'none': Suppresses the legend display.

matlab
legend('sin(x)', 'cos(x)', 'Location', 'northwest');

• ‘Position’ Property: For more precise control, you can directly manipulate the legend’s position using the ‘Position’ property. This property expects a four-element vector [left bottom width height], where the values are normalized to the figure’s dimensions.

matlab
legend('sin(x)', 'cos(x)');
leg = legend;
leg.Position = [0.2 0.8 0.2 0.1];

• Interactive Placement: You can interactively drag and resize the legend within the figure window by enabling legend’s interactive mode:

matlab
legend('sin(x)', 'cos(x)', 'AutoUpdate','off'); % Prevent automatic updates during dragging
leg = legend;
leg.Draggable = 'on'; % or 'all' to allow resizing as well

3. Optimizing Legend Appearance for Readability:

Besides placement, several formatting options enhance legend readability:

• Font properties: Control font size, style, and color using the 'FontSize', 'FontName', and 'TextColor' properties.

matlab
leg.FontSize = 12;
leg.FontName = 'Arial';
leg.TextColor = [0.5 0.5 0.5];

• Title: Add a descriptive title to the legend using the 'Title' property.

matlab
leg.Title.String = 'Trigonometric Functions';

• Background and Edge: Customize the legend’s background color, transparency, and edge properties using 'Color', 'Box', and 'EdgeColor'.

matlab
leg.Color = [0.95 0.95 0.95];
leg.Box = 'on';
leg.EdgeColor = 'k';

• Orientation: Arrange legend entries horizontally or vertically using the 'Orientation' property.

matlab
leg.Orientation = 'horizontal';

• Number of Columns: For long lists of entries, arrange the legend in multiple columns using the 'NumColumns' property.

matlab
legend('Data 1', 'Data 2', 'Data 3', 'Data 4', 'NumColumns', 2);

4. Advanced Legend Placement Techniques:

• Placing Legend Outside the Axes: When space within the axes is limited, placing the legend outside can be advantageous. This requires adjusting the figure’s outer position to accommodate the legend.

“`matlab
ax = gca;
outerpos = ax.OuterPosition;
ti = ax.TightInset;
left = outerpos(1) + ti(1);
bottom = outerpos(2) + ti(2);
ax_width = outerpos(3) – ti(1) – ti(3);
ax_height = outerpos(4) – ti(2) – ti(4);
ax.Position = [left bottom ax_width ax_height];

legend(‘Data 1’, ‘Data 2’, ‘Location’, ‘eastoutside’);
“`

• Using Normalized Units for Consistent Positioning: Using normalized units ensures consistent legend placement across different figure sizes and resolutions.

matlab
fig = gcf;
fig.Units = 'normalized';
ax = gca;
ax.Units = 'normalized';
legend('Data 1', 'Data 2', 'Position', [0.7 0.8 0.2 0.1], 'Units', 'normalized');

• Programmatically Determining Optimal Placement: For complex plots with numerous overlapping elements, you can develop algorithms to programmatically search for the optimal legend position based on whitespace analysis or other criteria.

• Integrating Legends with Subplots: When working with subplots, handle each subplot’s legend individually. You can create a single legend for all subplots if they share the same data series, or individual legends for each subplot.

“`matlab
subplot(2,1,1);
plot(x, sin(x));
legend(‘sin(x)’);

subplot(2,1,2);
plot(x, cos(x));
legend(‘cos(x)’);
“`

5. Considerations for Specific Plot Types:

• Scatter Plots: For scatter plots with numerous data points, consider using a smaller legend or placing it outside the axes to avoid obscuring data.
• Line Plots: Ensure the legend doesn’t overlap crucial data points or intersections.
• Bar Charts and Histograms: Position the legend strategically to avoid overlapping bars or bins.
• 3D Plots: Legend placement in 3D plots can be challenging. Experiment with different locations and transparency to ensure visibility.

6. Best Practices for Effective Legend Usage:

• Keep it Concise: Use clear and concise labels for legend entries. Avoid lengthy descriptions.
• Match Visuals: Ensure legend markers and colors accurately represent the corresponding data series in the plot.
• Maintain Consistency: Use consistent legend placement and formatting across multiple figures within a document or presentation.
• Consider the Audience: Adapt the legend’s appearance and placement based on the target audience and the complexity of the data being presented.

By carefully considering these strategies and best practices, you can elevate the effectiveness of your MATLAB figures and ensure clear and compelling communication of your data. Remember that the goal is to enhance understanding, and a well-placed and formatted legend plays a crucial role in achieving this objective. Experiment with different approaches and tailor your legend strategy to the specific needs of each visualization. The extra effort invested in optimizing legend placement will undoubtedly contribute to clearer, more impactful, and professionally presented data visualizations.