#12 graph: use a default -xdata parameter

[Andreas Drollinger]

The -xdata parameter is very often just a linear
sequence {0 1 2 3 4 ..}.

Blt::graph requires always the -xdata AND the -ydata
parameter to present properly a graph.

For simplicity reasons it would be nice to use a
default linear sequence for the -xdata parameter if
-xdata hasn't been specified.
The length of this default parameter should be
automatically adapted to the length of the -ydata
parameter.
A new parameter (f.ex: -xscale) could be used to
define the starting point and the step size of the
default -xdata paramter.