#1 Data streaming

[Jan van der Lee]

JPlot is designed to work with datafiles. It reads a datafile and
then uses data-arrays for plotting. Most properties (i.e. names of
data arrays etc.) are read from the file, not from the data array.

All properties should be assigned to data arrays. Then, we may
feed JPlot directly with data arrays, thus bypassing any file.

For this, the DataArray class must be rewritten.