This patch allows assigning names to data sets
in data files and selecting them during a plot
by use of their name with the index modifier.
Sometimes it is inconvenient to work with index
numbers to select data sets from data files.
Especially human readability can be increased
by "named" data sets and data sets can be stuffed
into a data file at every position if they can be
accessed by a name instead of a number (their
Names are assigned to data sets in comment lines:
The requested name from the plot command is compared
with the comment lines (comment char and leading
whitespace removed): If the line starts with the
requested string, the next data set will be plotted.
plot '-' index 'Population'
# Something else here