Plotting

The plot function produces two-dimensional plots. Many different combinations of arguments are possible. The simplest form is

plot (y)

where the argument y is taken as the set of y coordinates and the x coordinates are taken to be the indices of the elements, starting with 1. If both x and y are vectors, the elements of y are plotted versus the elements of x. For example,

> z = -3:0.2:5;
> w = 2*z.^3 - 3*z.^2 - 5*z + 1;
> plot(z,w)

plots the above cubic polynomial curve on the xy-plane.

For Matlab users: It has many built-in functions to deal with polynomials. For example, in order to get the values for a polynomial function we could use their polyval() in the following way.

> z = -3:0.2:5;
> w = polyval([2 -3 -5 1], z);

Plotting options. If more than one argument is given, they are interpreted as

plot (x, y, 'r-')

where y and 'r-' are optional. The format argument 'r-', if present, is interpreted as follows.

• '-' sets solid line plot style (default).
• '--' sets dashed line plot style.
• '.' sets dots plot style.
• If it is one of 'r', 'g', 'b', 'm', 'c', or 'w', it is interpreted as the plot color (red, green, blue, magenta, cyan, or white).
• If it is one of '+', '*', 'o', 'x', it will set the point style.

For more options and overall information, type

help plot

It can “hold” the current data on the plot when executing subsequent plotting commands. This allows us to execute a series of plot commands and have all the lines end up on the same figure. The default is for each new plot command to clear the plot device first. The command

hold on

turns the hold state on. And the command “hold off” turns the hold state off. Or, you can clear the current figure by typing

clf

While you “hold on” the current graph, you can specify the axis limits via

axis([x_lower,x_upper])

or

axis([x_lower,x_upper,y_lower,y_upper])

with lower, upper limit x_lower, x_upper for -axis, and lower, upper limit y_lower, y_upper for -axis. The command “grid on” is also useful, displaying the grid on the plot (and “grid off” removes the grid off the plot). Other useful commands for plotting are as follows:

• title('Title of the plot') creates a title at the top of graph.
• xlabel('x axis') and ylabel('y axis') specify x and y axis labels for the plot.
• legend('1st plot', '2nd plot', ..., pos) puts a legend on the current plot using '1st plot', '2nd plot',... as labels for all the plots before calling legend. The value pos optionally places the legend in the specified location:
  1. pos = -1: To the top right of the plot
  2. pos = 0: Don't move the legend box (default)
  3. pos = 1: Upper right-hand corner
  4. pos = 2: Upper left-hand corner
  5. pos = 3: Lower left-hand corner
  6. pos = 4: Lower right-hand corner

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