Joing multiple datasets for grouping in the graph key

If you want to group datasets in the graph key, e.g., to compare simulation
results with measurement data ... or some approximate results with results from
a more detailed calculation, you can pass a list of datasets to the
`graph.data.join` class.

When doing so, you have to assign unique names to the corresponding datasets.
In the example, we have used `x_a` and `y_a` as well as `x_b` and
`y_b` as names. Subsequently, one has to tell the graph style which data
columns—identified by the given name—it has to plot. This can be done
by inserting `graph.style.pos` instances before the respective graph style. By
passing a dictionary to the `usenames` argument, one specifies the
mapping from the data columns to the corresponding graph axes `x`, `y`, `x2`,
and so on.

! When using the standard names `x`, `y`, etc. for the data columns, as is done
in most cases, PyX does insert the `graph.style.pos` instance mapping those
data columns to the axis with the same name, automatically.
