Naming bars

To name the bars you just need to use a proper data column (instead of column
0, which is the line number in `data.file` instances). ... Beside that we show
in this example how to adjust the bar axis painter to properly show the names.

! The internals of a bar axis are quite different from a continuous axis (like
a linear or logarithmic) axis. A bar axis just handles a set of subaxes - one
for each discrete value. The painting of a bar axis can only draw the names for
the subaxes and some ticks between the subaxes. Other axes features known from
continous axes like ticks and labels, partitioners, texters, raters are
non-existent for bar axes.
