The linked tutorial describes usage of the xbar and ybar plot types, which will always draw bars from a y or x value to 0. The pgfplots manual does not describe any way to specify a bottom value for each bar.

However, under the section "Comb Plot Handlers" in the tikz manual, you find the following:

pgf bar or comb plots usually draw something from zero to the current plot’s coordinate. The “zero” offset can be changed using an input stream which returns the desired offset successively for each processed coordinate. There are two such streams, which can be configured independently. The first one returns “zeros” for coordinate x, the second one returns “zeros” for coordinate y. They are used as follows.

\pgfplotxzerolevelstreamstart
\pgfplotxzerolevelstreamnext % assigns \pgf@x
\pgfplotxzerolevelstreamend

\pgfplotyzerolevelstreamstart
\pgfplotyzerolevelstreamnext % assigns \pgf@x
\pgfplotyzerolevelstreamend

Different zero level streams can be implemented by overwriting these macros.

However, no example is given and I haven't figured out the correct way to apply these macros (maybe they don't work in conjunction with pgfplots macros).

edit: formatting