Minimal bar-graph example

For a minimal bar plot you have set an bar axis in the graph constructor and
provide Xname column data (X stands for the bar axis to be used). ... Here, we
just use column 0 which is automatically filled by `graph.data.file` with the
line number of the corresponding entry. Furthermore, you need to specify the
graph style, since the default graph styles `symbol` and `function` (depending on
the data type) are not appropriate for bar graphs.

Note that bar graphs differ from other xy-graphs in that they use ''discrete
axes'' for one graph dimension. However, the only affected components of this
fundamental change are one of the axes, which needs to be come a discrete one,
i.e. a bar axis, and the usage of appropriate graph styles.

! A bar graph is fundamentally different from a graph with a histogram style in
its usage of a ''discrete axis'' in one graph dimension. A histogram instead is
created using ''continuous axes'' in all graph dimensions and just drawing the
data in a specific bar-graph-like presentation. In particular, the discreteness
of the bar axis is reflected in the naming of its column name: instead of the
continuous "X" it expects an "Xname" (where X stands for the bar axis used) as
mentioned above.

! As all axes and graph dimensions in the PyX graph system are treated equally
all you need to modify to get bar graph with horizontal bars is to assign the
bar axis to the y-axis in the graph constructor and change the names of the
data columns to `yname` and `x`.

!! By using the bar style you implicitly also select a different positioning
style, namepy `barpos`. This positioning style handles a single-nested axes
with sub-axis values of the range going from 0 to 1.
