Matlab plot function

Create a simple line plot and label the axes. Customize the appearance of plotted lines by changing the line color, the line style, and adding markers.

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Introduction to MATLAB Plot Function

MATLAB or Matrix Laboratory is a programming language that was developed by MathWorks. This powerful language finds its utility in technical computing. MATLAB provides us with a convenient environment that can be used to integrate tasks like manipulations on matrix, plotting data and functions, implementing algorithms, creating user interfaces, etc. MATLAB is also convenient as it gives the solutions in the form that its user can easily understand. It uses mathematical notations to display the solutions.

Where can MATLAB be used?

Below are a few areas where we can use MATLAB:

• Computation
• Development of Algorithms
• Modeling
• Simulation
• Prototyping
• Analysis and Visualization of data
• Scientific graphs
• Engineering graphics
• Developing applications

MATLAB comprises a number of techniques to perform the above-mentioned uses. The objective of this article is to have a thorough understanding of plot functions in MATLAB. As the name suggests, the purpose of the plot function is to plot the graph of a function in MATLAB. We use a plot function to create a graphical representation of our data. It is straightforward and easy to visualize a trend in data when it is plotted as compared to just looking at the raw figures.

Syntax of Plot Function

Below are the syntax of Plot Function:

• fplot (f)
• fplot (f, xinterval)
• fplot (funx, funy)
• fplot (___, LineSpec)
• fplot (___, Name, Value)
• fp = fplot (___)

Let us now understand all the above-mentioned functions one by one:

1. fplot (f)

This will plot the graph defined by the function y = f(x). The plot is created over the default interval [-5, 5].

Let’s understand this function with the help of an example:

Here we have taken square function as the one which we need to plot. The output of our example will be as shown below:

2. fplot (f, xinterval)

In the above example, the interval was defaulting, however, if we need the interval of our choice, we can use fplot (f, xinterval). We need to specify the interval as a 2-element vector in the form [xmin xmax]

Let’s understand this clearly with the help of an example:

Here we have taken the same function as in the example above. For this example, we will take the interval 0 to 3. The output of our example will be as shown below:

3. fplot (funx, funy)

This function will plot the 2 functions passed as arguments. Here we define 2 functions ‘at’ and ‘bt’ as follows:

The output of the final function fplot (at, bt), will be

4. fplot (___, LineSpec)

This function is used to get the marker symbol, line style or color of the plotline as per the choice of user. i.e user can decide how he/she wants to visualize the final output.

For example, if we need to plot a graph of red color, we will pass ‘r’ as the argument in any of the functions explained above.

Here is an example to understand this:

Example:

The output that we will get here will be a red color plot of square function:

Similarly, we can use ‘b’ for blue color and ‘y’ for yellow color.

5. fplot (__, name, value)

This function is used to define properties of the line in the plot as per the pair of name-value passed in the argument. For example, if we pass ‘Linewidth’, 3 as an argument in any of the above examples, we will get a plot of width 3.

Here is an example to understand this:

 6. fp = fplot (___)

This function is used to modify the properties of an existing plot. So, we can change the properties of a plot that is already created, by using this function.

Let us first draw plot for a sin wave using the function fp = fplot (@(x) sin(x))

Let us now understand various ways in which we can modify properties of our plot obtained from fp:

• To change the line of the plot into a line with colons, we can use fp.LineStyle = ‘:’;
• To change the color of the line, we can use fp.Color = ‘r’;
• To get marker points on the plot, we can use fp.Marker = ‘x’;
• To get the desired color of the markers, we can use fp.MarkerEdgeColor = ‘b’;

After implementing the above functions, our final output will be:

Conclusion

MATLAB is a system whose basic data element is an array that does not require any dimensioning. This allows us to solve computing problems, especially the problems with matrix & vector formulations. In addition to its computing powers, MATLAB also provides its users the ability to draw and visualize the data for better understanding and drawing inferences.

All this is done in a significantly less amount of time when compared to writing a program in a scalar and non-interactive language such as C.

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5. Complete Guide to Matlab Comet()
6. Guide to How to Use Matlab?
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The plot function usually takes two arguments (but can take one). The first is the X values of the points to plot, and the second is the Y value of the points to plot. Below is an example of creating and plotting the values of the X squared graph from -10 to +10. In the case of a single argument, the X axis becomes 1,2,3,4,... up to the length of the array, and the Y axis contains the values of the array.

Notice that plot will "connect the dots" for you, drawing lines between each set of X,Y values.

If you have a single array that you wish to plot, and don't really care about the X axis, you just want to see the values plotted in the Y axis, you can pass a single array to the plot function. Here is an example:

The following functions are almost always used with plot to make the output more readable.

1. xlabel, ylabel : Provide a labeling for the graph axii.
2. title : Provide a title for the graph.
3. legend: Provide a legend telling what multiple graph lines mean.
4. grid: Put a checkered grid over the graph (add more lines to (sometimes) make it easier to see values.
5. axis: Tell how big of an area the graph should display.

Sometimes you will want to "clear" the figure. This is normally done by simply "plotting" the next thing you want. Alternatively, you can use the "clf" command:

Sometimes you will want to plot multiple graphs on the XY axis (on the same figure). In Matlab, when you use the plot function more than once, each time you call it, the previous figure is "erased".

To make a previous figure remain on the plot, we use the "hold on;" command. When we want to resume clearing the figure for each new plot, we use the "hold off;" command.

Try the following code:

You should have only seen one graph, the cubed graph (which was the second plot above). Now try this code, using the "hold on;" command.

Sometimes you will want to place multiple plots side by side on a single figure. You can achieve this by using the Matlab subplot function.

For many more details on this function (as well as the commands and functions mentioned above) you can always use the help command at the Matlab prompt.

The parameters used by the subplot function determine how many rows and columns for the overall "matrix" of figures, and then which of these positions to put the next plot in. The parameters are: 1) The number of rows, 2) the number of columns, 3) which of the sub plots to use.

Here is an example of how to create a 2x3 matrix (6 figures) of plots and to address each one of them by a number (shown below). Notice how row 1, column 1, is identified by the number 1. Row one, column three is identified by 3. Row two, column one, "wraps" and is identified by 4.:

Test this code. Notice that the subplot command stays the same, except for the "index" into which matrix location you want. Also notice that you can title, label, etc, each plot.

Matlab provides several well known graphing functions, including histograms, pie charts, and stem charts.

1. Histograms: Histograms are used to show how many values in an array of values fall within certain "bins". For example, if we have the scores of all students on a test, we may want to find out how many scores were between 90-100, 80-90, 70-80, etc. The histogram is the perfect tool for this.

2. Pie Charts: Pie Charts are useful for showing what percentage of a whole, each element represents. For example, if we knew there were 40 hours in the week, we could use a pie chart to show how many hours were spent on study, sleep, play, etc

3. Stem Charts: The stem chart draws a line from the bottom of the graph to the appropriate Y value for each X value. For example if the X axis represents how many times a letter was found in a document, we could use a stem plot to show the relationship between all letters, and how often they occur..

To plot the graph of a function, you need to take the following steps −

• Define x, by specifying the range of values for the variable x, for which the function is to be plotted

• Define the function, y = f(x)

• Call the plot command, as plot(x, y)

Following example would demonstrate the concept. Let us plot the simple function y = x for the range of values for x from 0 to 100, with an increment of 5.

Create a script file and type the following code −

When you run the file, MATLAB displays the following plot −

Let us take one more example to plot the function y = x². In this example, we will draw two graphs with the same function, but in second time, we will reduce the value of increment. Please note that as we decrease the increment, the graph becomes smoother.

Create a script file and type the following code −

When you run the file, MATLAB displays the following plot −

Change the code file a little, reduce the increment to 5 −

MATLAB draws a smoother graph −

Adding Title, Labels, Grid Lines and Scaling on the Graph

MATLAB allows you to add title, labels along the x-axis and y-axis, grid lines and also to adjust the axes to spruce up the graph.

• The xlabel and ylabel commands generate labels along x-axis and y-axis.

• The title command allows you to put a title on the graph.

• The grid on command allows you to put the grid lines on the graph.

• The axis equal command allows generating the plot with the same scale factors and the spaces on both axes.

• The axis square command generates a square plot.

Example

Create a script file and type the following code −

MATLAB generates the following graph −

Drawing Multiple Functions on the Same Graph

You can draw multiple graphs on the same plot. The following example demonstrates the concept −

Example

Create a script file and type the following code −

MATLAB generates the following graph −

Setting Colors on Graph

MATLAB provides eight basic color options for drawing graphs. The following table shows the colors and their codes −

Example

Let us draw the graph of two polynomials

• f(x) = 3x⁴ + 2x³+ 7x² + 2x + 9 and

• g(x) = 5x³ + 9x + 2

Create a script file and type the following code −

When you run the file, MATLAB generates the following graph −

Setting Axis Scales

The axis command allows you to set the axis scales. You can provide minimum and maximum values for x and y axes using the axis command in the following way −

The following example shows this −

Example

Create a script file and type the following code −

When you run the file, MATLAB generates the following graph −

Generating Sub-Plots

When you create an array of plots in the same figure, each of these plots is called a subplot. The subplot command is used for creating subplots.

Syntax for the command is −

where, m and n are the number of rows and columns of the plot array and p specifies where to put a particular plot.

Each plot created with the subplot command can have its own characteristics. Following example demonstrates the concept −

Example

Let us generate two plots −

y = e^−1.5xsin(10x)

y = e^−2xsin(10x)

Create a script file and type the following code −

When you run the file, MATLAB generates the following graph −