Create 2D Plots

The versatile plot command is used to create 2D line and scatter plots.

Refer to the on-line reference, e.g. "doc plot" or "help plot", for more details on any of the commands listed below.

A Note on Graphics

MATLAB makes it easy to create plots from data sets. Although the default for each plot type is a good start, you should also consider whether and how adjustments to the defaults may alter the way that the graphical information is received by the viewer.

One of the most basic factors to consider is the scaling of the axes and whether zero should be included on the axis limits. We won't delve into these issues on this web page. The reader is urged to consider the arguments presented on these sites:

• Visualization: Misleading axes on graphs
• Wikipedia page on Misleading graphs
• Real life examples of misleading graphs from Statitisticshowto.com

MATLAB Commands

The basic commands for creating 2D plots in MATLAB are

• plot for creating 2D plots on linear axes
• semilogx, semilogy, loglog for creating 2D plots with logarithmic axes
• axis, xlim, ylim for changing the scale on the x and y axes
• xlabel, ylabel for adding axis labels
• legend for adding a legend when a 2D plot has more than one data set
• subplot for creating more than one plot in a figure window
• figure for opening more than one figure window at a time

plot: Create a two-dimensional plot

In each of the following statements, x and y are vectors with the same number of elements. In other words, length(x) and length(y) are equal.

>> plot(x,y)       %  Plot (x,y) pairs, connect points with lines
>> plot(x,y,'.')   %  Plot (x,y) pairs with dots at each point

If only two arguments are provided, the data pairs are connected with a solid black line.

The third argument is a string that specifies whether (or not) each data pair is identified with a symbol, whether (or not) the data pairs are connected by lines, and the line type if lines are used.

Here is an excerpt from the output returned by "help plot"

Various line types, plot symbols and colors may be obtained with
plot(X,Y,S) where S is a character string made from one element
from any or all the following 3 columns:
 
       b     blue          .     point              -     solid
       g     green         o     circle             :     dotted
       r     red           x     x-mark             -.    dashdot 
       c     cyan          +     plus               --    dashed   
       m     magenta       *     star             (none)  no line
       y     yellow        s     square
       k     black         d     diamond
       w     white         v     triangle (down)
                           ^     triangle (up)
                           <     triangle (left)
                           >     triangle (right)
                           p     pentagram
                           h     hexagram

Multiple sets of data pairs can be plotted with a single plot command.

Consider the following, where x and y are vectors with the same number of elements, and v and w are vectors with the same number of elements. In other words, length(x) and length(y) are equal, length(v) and length(w) are equal, but length(x) and length(v) are not necessarily equal.

>> plot(x,y,v,w)      %  Plot (x,y) pairs connected w/ lines
                      %    and (v,w) pairs connected with lines
                      
>> plot(x,y,'k--',v,w,'bo')  %  plot (x,y) pairs with the points
                             %    connected by dashed black lines;
                             %    and plot (v,w)pairs as points identified with blue circles

When x and y have a large number of points, say more than 50, and the plot(x,y) command is used, the plot tends to appear as a smooth curve. However, the plot is just a set of straight line segments connected pairs of data points. Consider the smooth plot of the sine function

x = linspace(0,2*pi);
y = sin(x);
plot(x,y)

If fewer points are used, a plot of the sine function clearly shows that plot(x,y) connects the data pairs with straight line segments

x = linspace(0,2*pi,7);
y = sin(x);
plot(x,y)

If any of the data pairs have NaN (not-a-number) values, the data pair is omitted from the plot

x = linspace(0,2*pi,7);
y = sin(x);
y(4) = NaN;
plot(x,y)

xlabel, ylabel: Add x and y labels to an existing plot

The xlabel and ylabel commands take a single argument, which is a character string.

>> x = ...;  y = ...;
>> plot(x,y)
>> xlabel('X values');
>> ylabel('y = f(x)')

The character string can contain LaTeX codes for Greek letters such as \alpha, \beta, \gamma, \theta. For example, the following statements create a plot of $sin(\theta)$ and label the axes with the \theta symbol.

x = linspace(0,2*pi);
y = sin(x);
plot(x,y);
xlabel('\theta')
ylabel('sin(\theta)')

Note that Greek symbols are only used to label output. All MATLAB variables are plain text.

For more information consult the help page on the Mathworks web site.

axis: Change the range of the x and y axes

The axis command changes the limits of both the x and y axes.

>> x = ...;  y = ...;
>> plot(x,y)
>> axis([-2 4 0 5])     %  Set x-axis limits to [-2, 4] and y-axis limits to [0, 5]

xlim, ylim: Change the range of the x or y axes

The xlim and ylim commands independently change the limits of the x and y axes.

>> x = ...;  y = ...;
>> plot(x,y)
>> xlim([-2 4] )     %  Set only the x-axis limits to [-2, 4]
>> ylim([0 5])       %  Set only the y-axis limits to [0, 5]

You may ask, "Why use two commands xlim and ylim when the axis command does the same thing in just one command? Well, if you want to change the scale of both the x axis and the y axis, it makes sense to use the axis command. However, there are situations where you only want to control one axis scale and let MATLAB automatically decide the other axis scale. In those situations, it makes sense to use xlim or ylim.

subplot: Put multiple plots in a single figure window

The following example shows how to create two subplots, one above the other.

>> subplot(2,1,1)
>> plot( ... )       % some 2D plot 
>> subplot(2,2,1)
>> plot( ... )       % another 2D plot

Note that subplot has two roles. First, it tells MATLAB to put multiple plots in an array, and second it tells MATLAB which array location to use for the next plot.

subplot(m,n,1): The next plot is the first plot in the m by n array of plots.

subplot(m,n,2): The next plot is the second plot in the m by n array of plots.

See also the figure command, below.

close: Close open plot windows

>> close('all')     % close all open plot windows
>> close all

The close('all') command can be a convenient way to cause MATLAB to open a new plot window on top of the command window and any open editor windows. For example, suppose a function called make_my_plot creates a plot. If the following composite (two commands on one line) command is entered, any open plot windows are closed and a new plot window will be drawn on top of any open MATLAB windows.

>> close all;  make_my_plot

figure: Open a new plot window

>> figure                    % open a new plot window
>> figure('Name','My data')  % open a new plot window with
                             %   'My data' in the title bar

Using named windows can be helpful when many plot windows are opened by a single m-file.

The figure command allows multiple plot windows to be open at the same time. The following example shows how two figure windows could be opened and each given a descriptive name in the title bar.

>> figure('Name','Plot of approximation to f(x)')
>> plot(x,y)
>> figure('Name','Plot of error in approximation')
>> plot(x,y-yexact)

Using the figure command to open a new plot window should not be confused with using the subplot command to put multiple plots in the same window.