label docs

[James R. V. Z. <jr...@co...>]

I remember seeing there was now a way label each data point with a
string read from the data file, but I couldn't find the command in the
help file.  Eventually I resorted to running demo scripts until I
found an example.  I just added some cross-references in gnuplot.doc.

In the process, I discovered that the title in the key could come from
a column heading in the data file.  That wasn't mentioned under "help
plot title", so I added a cross reference for that, too.  I thought
about changing the syntax example from

       title "<title>" | notitle ["<ignored title>"]
to
       title "<title>" | title <columnnumber> | notitle ["<ignored title>"]

Should I?

        - Jim Van Zandt