Plots in Matlab

From LiteratePrograms

A lot of new users ask about how to plot the graph of a function in MATLAB, here are some simple examples.

Classic plot

result of the simple plot

If you want to draw the plot of a function, you first need to define your function. MATLAB provide anonymous function definition to do that.

We will first try something with:

<<define an anonymous function>>=
f=@(x)((x+1)./(x.*sin(x)+1))

notice the 'dot' before the operators, it's because we want to perform a pointwise application of our function.

now we have to define about the domain on which we want to plot $f$ , let's define it through its bounds, and define the resolution of our plot:

<<domain and resolution>>=
x_bounds     = [-3, 3];
x_resolution = .1;
x_domain     = (x_bounds(1):x_resolution:x_bounds(2))'

Then we can apply it by simply calling $f$ on |x_domain|:

<<apply f>>=
y = f(x_domain);

we just have to plot it:

<<plot the result>>=
figure;
plot(x_domain,y)

we decided to open a figure (we can add options to choose color, menu, etc) before really plotting the graph.

The plot function has options too, as in:

plot(x_domain,y,'linewidth',2);

which is easier to see on a projector.

now the whole code for downloading purpose:

<<example_of_plot_01.m>>=
define an anonymous function
domain and resolution
apply f
plot the result

Plot with legend

Following a question on the MATLAB newsgroup, here is a standard way to build custom legends.

Generation of data

I first generate fake datasets to be used in my example.

The first one is a 2-dimensional exponential of Brownian motion
The second one is a 2-dimensional process whose increasings are uniformly distributed

<<fake datasets>>=
data1 = struct('date', (1:100)', 'value', 100*exp(cumsum(randn(100,2))/10), 'names', {{ 'line 1', 'line 2'}}, 'title', 'dataset 1')
data2 = struct('date', (1:100)', 'value', 100*cumsum(rand(100,2)*2-1+.1), 'names', {{ 'line 1', 'line 2'}}, 'title', 'dataset 2')

My dataset have a structured shape (read more on structured datasets in MATLAB), like this :

data1 = 
    date: [100x1 double]
   value: [100x2 double]
   names: {'line 1'  'line 2'}
   title: 'dataset 1'

This is quite useful to have such container which always includes :

values
corresponding dates
names of the columns (variables)
name of the dataset (i.e. title)

Plot and legend

Image:Legend-plot.png

As you can see, the plot is quite standard but I demand an output which gives me handles on plotted lines.

Then, I can ask to the |legend| MATLAB function to only address those lines.

Notice the way I generate the legend which is a nice use of cellfun...

<<example_of_plot_with_legend_01.m>>=
fake datasets
%% plot
% notice the handle I get back from the plots
figure;
h1 = plot(data1.date, data1.value, 'linewidth',2)
hold on
h2 = plot(data2.date, data2.value, ':', 'linewidth',2)
hold off
%% Legend
legend(gca,[h1;h2], cellfun(@(x,y)([x ' ' y]), ...
        {data1.names{:}, data2.names{:}}', ...
        cellstr( [repmat(data1.title, length(data1.names),1); ...
                  repmat(data2.title,length(data2.names),1)]), ...
        'uniformoutput',false), 'Location',  'NorthWest' )