[org-plot] multiple graphs on a plot

[Eric S Fraga <ucecesf@ucl.ac.uk>]

Back in September, zwz asked about getting multiple graphs on a plot
from a table:

http://lists.gnu.org/archive/html/emacs-orgmode/2009-09/msg00354.html

There appears to be no answer to that question.  I have the same need
and I was wondering if anybody has an answer to this.  

Basically, given a table that looks like this:

| x   | y1 |   x | y2 |
| 1   |  2 | 1.5 |  3 |
| ... |    |     |    |

I want to plot y1 versus x as one line on a plot and y2 versus x as
another line on the same plot.  In gnuplot, this is straightforward:

  plot ... using 1:2, '' using 3:4

but org-plot currently assumes :ind to have just one entry.  I have
started to look at the code in org-plot to see how to generalise ind
to be a vector of columns but would rather avoid the work if somebody
else has already done this (and because although my elisp skills are
improving daily due to org-mode, they are still rather rudimentary
;-).

Thanks,
eric