How to draw probability density function in MatLab?

You can generate a discrete probability distribution for your integers using the function hist:

data = [1 2 3 3 4];           %# Sample data
xRange = 0:10;                %# Range of integers to compute a probability for
N = hist(data,xRange);        %# Bin the data
plot(xRange,N./numel(data));  %# Plot the probabilities for each integer
xlabel('Integer value');
ylabel('Probability');

And here's the resulting plot:

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UPDATE:

In newer versions of MATLAB the hist function is no longer recommended. Instead, you can use the histcounts function like so to produce the same figure as above:

data = [1 2 3 3 4];
N = histcounts(data, 'BinLimits', [0 10], 'BinMethod', 'integers', 'Normalization', 'pdf');
plot(N);
xlabel('Integer value');
ylabel('Probability');

If you want a continuous distribution function, try this.

x = [1 2 3 3 4]
subplot(2,1,1)
ksdensity(x)
axis([-4 8 0 0.4])

subplot(2,1,2)
cdfplot(x)
grid off
axis([-4 8 0 1])
title('')

Which outputs this.

The Cumulative Distribution Function is on the bottom, the Kernel Density Estimate on the top.

type "ksdensity" in matlab help and you will find out the function that will give you the continuous form of PDF. I guess this is exactly what you are looking for.