06 Data Visualization & Plotting

Overview: The Graphics Engine

MATLAB features a graphics rendering system based on Handle Graphics. This note covers 2D line plots, tiled layouts, 3D meshes, and handle property modifications.

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1. 2D Plotting and Graph Labeling

The fundamental tool for 2D graphing is plot.

Function Signatures:

• plot
  • Syntax: plot(Y), plot(X, Y), or plot(X, Y, lineSpec)
  • Parameters:
    • X: Independent variable data vector.
    • Y: Dependent variable data vector.
    • lineSpec (Optional): Formatting string defining color, marker, and line style (e.g. 'r--' for red dashed line).
• axis
  • Syntax: axis([xmin, xmax, ymin, ymax]), axis equal, or axis square
  • Description: Sets axis limits or adjusts coordinate ratios.
• legend
  • Syntax: legend(label1, label2, ...)
  • Description: Adds a labels legend to curves in the order they were plotted.

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2. Dynamic Plotting Methods: hold and fplot

Plotting Multiple Curves:

By default, calling plot wipes the active figure window. To plot multiple curves, you can:

1. Pass multiple sets of vectors: plot(x, y1, x, y2)
2. Use hold on to keep the current plot and overlay new curves. Call hold off when finished.

Functional Plotting:

• fplot
  • Syntax: fplot(f, limits)
  • Parameters:
    • f: A function handle (e.g. @(x) sin(x)).
    • limits: Vector setting domain bounds [xmin, xmax].
  • Description: Dynamically samples and plots a mathematical function over the specified interval.

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3. Subplots (Tiled Layouts)

To display multiple plots side-by-side in a single window, partition the screen layout.

• subplot
  • Syntax: subplot(m, n, p)
  • Parameters:
    • m: Total row divisions.
    • n: Total column divisions.
    • p: The index slot of the active tile (numbered row-by-row, left-to-right).

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4. Specialized 3D Surface Plots

• plot3
  • Syntax: plot3(X, Y, Z)
  • Description: Plots a 3D line trajectory.
• mesh / contour / pcolor
  • Syntax: mesh(Z) (3D grid), contour(Z) (2D isolines), pcolor(Z) (2D colored matrix cells).
  • Description: Generates surface or density plots from a grid matrix Z.

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5. Handle Graphics (set, get, gca)

Every graphical element (figure, axes, lines) is an object with a unique identifier handle.

Key Handle Functions:

• gca (Get Current Axes)
  • Syntax: ax = gca
  • Description: Returns the handle of the current axes.
• gcf (Get Current Figure)
  • Syntax: fig = gcf
  • Description: Returns the handle of the current figure window.
• set
  • Syntax: set(handle, 'PropertyName', propertyValue)
  • Description: Modifies properties of a graphics object.
• get
  • Syntax: val = get(handle, 'PropertyName')
  • Description: Returns the current state value of a property.

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💻 Conceptual Code Demonstration

% 1. Sample data
x = linspace(0, 2*pi, 100);
y1 = sin(x);
y2 = cos(x);
 
% 2. Partition figure using subplot
subplot(1, 2, 1); % Selects first tile in 1x2 grid
plot(x, y1, 'b-');
title('Sine Wave');
 
subplot(1, 2, 2); % Selects second tile
lineHandle = plot(x, y2);
title('Cosine Wave');
 
% 3. Modify line using Handle Graphics properties
set(lineHandle, 'Color', 'r');      % Changes color to red
set(lineHandle, 'LineWidth', 2.0);  % Sets line thickness
set(lineHandle, 'LineStyle', '--');  % Sets dashed style
 
% Adjust axes direction using gca
set(gca, 'XLim', [0, 2*pi]);