Plotting in Matlab

A.J. Melhus, 4-18-10;
Revised by C. Chen, 2-17-12
Revised by H. Sheets, 3-21-13

Contents

Using plot command

The basic syntax is:

plot(x,y)

where x and y are vectors containing the data you wish to plot. x and y must have the same length.

To try, we should set up x and y first. Let's define a vector from 0 to 2pi, using the default stepsize of 1:

x1 = 0:2*pi
x1 =

     0     1     2     3     4     5     6

Then we can assign values to y1 based on x1:

y1 = sin(x1)
y1 =

         0    0.8415    0.9093    0.1411   -0.7568   -0.9589   -0.2794

Then we can plot the sine function:

plot(x1,y1)

THIS PLOT LOOKS BAD because x1 only contains 7 points (we need more points to make a smooth graph).

Using linspace command

Let's use linspace to make a much more smooth graph by creating a vector from 0 to 2pi containing 100 linearly spaced points:

x2 = linspace(0, 2*pi);
y2 = sin(x2);
plot(x2,y2)

THIS PLOT LOOKS GOOD because x2 contains many more points, creating a smoother graph. By default, linspace uses 100 points, but remember, if you give linspace a third argument, such as:

x3 = linspace(0,2*pi,50);

you can specify the number of points (such as 50) you want it to create.

Plot aesthetics

(taken/edited from http://www.engin.umich.edu/group/ctm/extras/plot.html )

The color and point marker can be changed on a plot by adding a third parameter (in single quotes) to the plot command: plot(x,y, 'aesthetic')

- aesthetic consists of one to three characters which specify a color and/or point marker type - color and point marker types include:

y yellow . point
m magenta o circle
c cyan x x-mark
r red + plus
g green - solid
b blue * star
w white : dotted
k black -. dashdot
-- dashed

For example, to plot y2 as a red, dotted line instead of the default blue straight line, the command should be changed to:

plot(x2,y2, 'r:')

title and labels

Whenever you make a plot, don't forget to properly label the x- and y-axis by using xlabel and ylabel:

plot(x2, y2)
xlabel('variable x')
ylabel('sin(x)')

Also, it's nice to give your plot a title so everyone can understand immediately what's the figure about:

title('sine function')

Note that the title and axis labels will be erased when you call plot again without using hold on.