5.3.1The plot function

Both plot() and plot2d() create 2D plots. plot() is borrowed from Matlab. Persons with Matlab experience may experience may want to use it the benefits of plot2d() are doubtful.

Scilab has the added plot2d() function. It offers more options to tailor the plot. Multiple plots, for instance (recall however that at multiple plots were done with plot().

The plot2d function has different forms, depending on the input arguments. If y is a vector, plot2d(y) produces a piecewise linear graph of the elements of y versus the index of the elements of y. If you specify two vectors as arguments, plot2d(x,y) produces a graph of y versus x.

Plot2d(x,y,[xcap,ycap,caption])

For example, these statements use the colon operator to create a vector of x values ranging from zero to 2π, compute the sine of these values, and plot the result. The characters %pi create the symbol π.

-->x = 0:%pi/100:2*%pi;

-->y = sin(x);

-->plot2d(x,y)

Now label the axes and add a title.

-->plot(x,y,’X’,’Y’,’Plot function y=sin(x)’)

And add the grid.

-->xgrid(5);

Figure 5.1 – Plot of the Sine function

Function xgrid([style]) adds a grid on a 2D plot. style is the dash id or the color id to use for the grid plotting. Style is an integer.

Multiple Data Sets in One Graph

Multiple x-y pair arguments create multiple graphs with a single call to plot. SCILAB automatically cycles through a predefined (but user settable) list of colors to allow discrimination among sets of data. For example, these statements plot three related functions of x, each curve in a separate distinguishing color.

>>y2 = sin(x-.25);

>>y3 = sin(x-.5);

>>plot(x,y,x,y2,x,y3)

The legend command provides an easy way to identify the individual plots.

>>legend('sin(x)','sin(x-.25)','sin(x-.5)')

Figure 5.2 – Multiple Data Sets in One Graph

Plots of the Parameter Functions

For example, these statements plot function y(t)=0.7cos(t) of x(t)=0.5sin(t), where parameter t € [0, 2π]:

>>t=[0:0.01:2*pi];

>>x=0.5*sin(t);

>>y=0.7*cos(t);

>>plot(x,y)

Plot of the sectionally given function

It is given

These statements plot of the sectionally given function:

>>x1=[-2*pi:0.01:-pi];

>>y1=pi*sin(x1);

>>x2=[-pi:0.01:pi];

>>y2=pi-abs(x2);

>>x3=[pi:0.01:2*pi];

>>y3=pi*sin(x3).^3;

>>x=[x1 x2 x3];

>>y=[y1 y2 y3];

>>plot(x,y)

or

>>plot(x1,y1,x2,y2,x3,y3)

5.3.2 The fplot function

fplot(<nameFunction> ,limits) - produces a graph of function in an interval . Where

<nameFunction> - name of a m-file as:

@ MyFun or 'MyFun'

or a line of a kind:

'sin(x)',

'[sin(x) cos(x) ]',

'[sin(x), myfun1(x), myfun2(x)]'.

limits=[xmin xmax ymin ymax] – defines intervals for values of argument and function.

For example:

fplot(@humps,[0 1]);

fplot('humps',[0 1]);

fplot('[tan(x),sin(x),cos(x)]',2*pi*[-1 1 -1 1]);

fplot('sin(1 ./ x)', [0.01 0.1],1e-3).

5.3.3 Preparing Graphs for Presentation

Specifying Line Styles and Colors

It is possible to specify color, line styles, and markers (such as plus signs or circles) when you plot your data using the plot command.

plot(x,y,'color_style_marker')

color_style_marker is a string containing from one to four characters (enclosed in single quotation marks) constructed from a color, a line style, and a marker type:

• color strings are 'c', 'm', 'y', 'r', 'g', 'b', 'w', and 'k'. These correspond to cyan, magenta, yellow, red, green, blue, white, and black;

• linestyle strings are '-' for solid, '--' for dashed, ':' for dotted, '-.' For dash-dot. Omit the linestyle for no line;

• the marker types are '+', 'o', '*', and 'x' and the filled marker types are 's' for square, 'd' for diamond, '^' for up triangle, 'v' for down triangle, '>' for right triangle, '<' for left triangle, 'p' for pentagram, 'h' for hexagram, and none for no marker.

You can also edit color, line style, and markers interactively.

Plotting Lines and Markers

If you specify a marker type but not a linestyle, SCILAB draws only the marker. For example,

plot(x,y,'ks')

plots black squares at each data point, but does not connect the markers with a line.

The statement

plot(x,y,'r:+')

plots a red dotted line and places plus sign markers at each data point. You may want to use fewer data points to plot the markers than you use to plot the lines. This example plots the data twice using a different number of points for the dotted line and marker plots.

>>x1 = 0:pi/100:2*pi;

>>x2 = 0:pi/10:2*pi;

>>plot(x1,sin(x1),'r:',x2,sin(x2),'r+')

Figure 5.3 - This example plots the data twice using a different number of points for the dotted line and marker plots

Figure command

The figure command creates a new figure object using default property value:

>>x1=[-2*pi:0.01:-pi];

>>y1=pi*sin(x1);

>>x2=[-pi:0.01:pi];

>>y2=pi-abs(x2);

>>x3=[pi:0.01:2*pi];

>>y3=pi*sin(x3).^3;

>>figure;plot(x1,y1);grid on;

>>figure;plot(x2,y2);grid on;figure;plot(x3,y3);grid on

Adding Plots to an Existing Graph

The hold command enables you to add plots to an existing graph. When you type

>>hold on

SCILAB does not replace the existing graph when you issue another plotting command; it adds the new data to the current graph, rescaling the axes if necessary.

>>x1=[-2*pi:0.01:-pi];

>>y1=pi*sin(x1);

>>x2=[-pi:0.01:pi];

>>y2=pi-abs(x2);

>>x3=[pi:0.01:2*pi];

>>y3=pi*sin(x3).^3;

>>figure;plot(x1,y1);hold on;plot(x2,y2);hold on;plot(x3,y3);grid on

Create axes in tiled positions

Command subplot(m,n,p), or subplot(mnp), breaks the Figure window into an m-by-n matrix of small axes, selects the p-th axes for for the current plot, and returns the axis handle. The axes are counted along the top row of the Figure window, then the second row, etc. For example,

>> figure; subplot(3,1,1); plot(x1,y1); grid on

>> subplot(3,1,2); plot(x2,y2); grid on

>> subplot(3,1,3); plot(x3,y3) ;grid on

Figure 5.4 - Illustration shows 3 subplot regions and indicates the command used to create each

Controlling the Axes

The axis command supports a number of options for setting the scaling, orientation, and aspect ratio of plots. You can also set these options interactively.

Setting Axis Limits

By default, SCILAB finds the maxima and minima of the data to choose the axis limits to span this range. The axis command enables you to specify your own limits

axis([xmin xmax ymin ymax])

or for three-dimensional graphs,

axis([xmin xmax ymin ymax zmin zmax])

Setting Axis Visibility

You can use the axis command to make the axis visible or invisible.

axis on

makes the axis visible. This is the default.

axis off

makes the axis invisible.

Setting Grid Lines

The grid command toggles grid lines on and off. The statement

grid on

turns the grid lines on and

grid off

turns them back off again.

Axis Labels and Titles

The xlabel, ylabel, and zlabel commands add x-, y-, and z-axis labels. The title command adds a title at the top of the figure and the text function inserts text anywhere in the figure. A subset of TeX notation produces Greek letters. You can also set these options interactively.

t = -pi:pi/100:pi;

y = sin(t);

plot(t,y)

axis([-pi pi -1 1])

xlabel('-\pi \leq {\itt} \leq \pi')

ylabel('sin(t)')

title('Graph of the sine function')

text(1,-1/3,'{\itNote the odd symmetry.}')

Figure 5.5 – Axis Labels and Titles

Saving a Figure

To save a figure, select Save from the File menu. To save it using a graphics format, such as TIFF, for use with other applications, select Export from the File menu. You can also save from the command line—use the saveas command, including any options to save the figure in a different format.