More on Plotting - Part 3

ezplot

ezplot   is an easy to use function plotter. The limits of the plot will be automatically set by the MATLAB when you run the command.

>> ezplot('sin(t)')

You can set the limits of the function;

ezplot(‘function’,[lower_limit upper_limit]).

The command plots the function between the limits specified.

>> ezplot('sin(t)',[0 4*pi])

Ezplot can be used to plot curve between two functions.

ezplot(‘function1’,’function2’, [lower_limit upper_limit])

The command plots function1(t) versus function2(t)  over lower_limit < t < upper_limit.

>> ezplot(’cos(t)’, ’sin(t)’, [0 2*pi]); axis square

You may also use a function handle to an existing function. Function handles are more powerful and efficient than string expressions.

>> ezplot(@cos,@sin)

The command will give same plot as ezplot(’cos(t)’, ’sin(t)’, [0 2*pi]).

(About function handles we will discuss later)

Contour Plots

contour(Z) draws a contour plot of matrix Z in the x-y plane, with the x-coordinates of the vertices corresponding to column indices of Z and the y-coordinates corresponding to row indices of Z. The contour levels are chosen automatically.

A contour plot of a function of two variables is a plot of the level curves of the function, that is, sets of points in the x-y plane where the function assumes a constant value. For example, the level curves of x2 + y2 are circles centered at the origin, and the levels are the squares of the radii of the circles.

We can make a contour plot of x2 + y2 as follows:

>> [x y] = meshgrid(-1:0.1:1, -1:0.1:1); % meshgrid is used to make grid of defined points.

>> contour(x, y, x.ˆ2 + y.ˆ2)

Field Plots

The MATLAB routine quiver is used to plot vector fields or arrays of arrows. The arrows can be located at equally spaced points in the plane (if x and y coordinates are not given explicitly), or they can be placed at specified locations.

quiver(U,V) plots velocity vectors at equally spaced points in  the x-y plane.

The following code, for example, plots a vector field similar to the magnetic field near a magnetic dipole.

>> [x,y] = meshgrid(-1:.1:1,-1:.1:1);

>> z=x.*exp(-x.^2 - y.^2);

>> [a b] = gradient(z);

>> quiver(a,b)

  

Scatter Plot

scatter(x, y, size, color) displays colored circles at the locations specified by the vectors x and y (which must be the same size).  ‘Size’ determines the size of each circle marker and ‘color’ defines the color of the marker.

scatter(x, y)  will plot the markers with default size and color.

Size can be a scalar or a vector the same length as x and y. If size is a scalar, MATLAB draws all the markers the same size. If it is empty, the default size is used. If size is a vector, each marker will take a size corresponding to value specified in the vector. Color also may be a vector, then the values in color vector are linearly mapped to the colors in the default colormap (colormap is RGB color lookup table of MATLAB).

>> x = -1:0.1:1;

>> y = x.^2;

>> s = 1:21;

>> c = s;

>> scatter(x,y,s,c)

Compass Plot

compass(U,V) draws a graph that displays the vectors with components (U,V) as arrows starting from the origin.

compass(Z) is equivalent to compass(REAL(Z),IMAG(Z)).

This plot is very useful to represent the phasors.

>> v=3+4i

>> i=2-1i

>> compass(v)

>> hold on

>> compass(i)

Area Plot

Area plot is used to plot a filled 2D plot. The graph displays elements in Y as one or more curves and fills the area under each curve. The plot will be same as the ‘plot’ command, but the area between the x axis and the curve will be filled in with color.

>> x=0:10;

>> y=2.*x-1;

>> area(x,y)

>> xlabel('x value')

>> ylabel('y values')

>> title('area plot example')

Filled Shapes

fill(x, y, 'c') command can be used to draw filled polygons. The function plots a 2D polygon defined by the column vectors x and y and fills with the color specified by c. The vertices of the polygon are specified by the pairs (xi, yi). If necessary, the polygon is closed by connecting the last vertex to the first. Like the plot function, fill can have any number of pairs of vertices and associated colors.

>> x=[-1 0 1];

>> y=[0 1 0];

>> fill(x,y,'r')

Bar Graph

bar(x, y) or bar(y) command is used to plot the bar graph. It plots bars for each value of x, the height as y. If only one vector as argument, the graph will be plotted against the length of the vector.

bar(x,y, width) or bar(y, width) specifies the width of the bars. Values of width > 1 produce overlapped bars.  The default value is 0.8

>> x=0:10;

>> y=2.*x-1;

>> bar(x,y)

>> title('bar plot example')

  

Histogram Plot

Histogram plots are used to represent the distribution of a variable with in a range.

hist(z) command produces a histogram bar plot of values of z for a default 10 division of toral range.

hist(z, n) produces a histogram bar plot of values of z for n equal divisions

>> z=rand(1,100)

>> hist(z)

>> xlabel('z value')

>> ylabel('frequency of z')

>> title('histogram example')

histogram(z) creates a histogram plot of z. The histogram function uses an automatic binning algorithm that returns bins with a uniform width, chosen to cover the range of elements in z.

Pie Charts

Standard pie charts can be created by using the pie(x) function, where x is a vector of values.

pie(x, b) used to plot pie chart and pull out a specific section of the pie chart, where b is the logical vector describing a slice or slices to be pulled out of the pie chart. The pie3 function is used to draw the pie chart with a 3-D appearance.

>> x=rand(1,6);

>> pie(x)

>> title('Example Pie Chart')

>> pie(x,x==max(x))    % pulls out the slice of maximum share

>> title('Example Pie Chart')