create a statistical distribution?

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I'm making a site mockup, and I have some stacked bar charts. I'd like
to populate them with dynamic data, instead of hard-coded data. I
don't want to use random data, because it winds up looking too
different from what the actual data will actually look like after

The stacked bar charts have 5 sections, which all together will add up
to 100%. An example distribution might be:
1 => 2%
2 => 3%
3 => 50%
4 => 20%
5 => 25%

But I'd like to constrain the data. Typically, 1 and 2 are less than
5%, 3 is below 20%, and the rest is split between 4  and 5 .

Is there a package that someone has written where I can specify these
constraints and get back a sample data that would fit a distribution I
would specify?

I can probably think of a way to do this using the rand() function,
but I would really like to see the "right" statistical way to do it.

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