Here is an example of feeding query output into gnuplot without leaving psql:

  # psql -U postgres
  psql (8.4.15)
  Type "help" for help.

  postgres=# \t
  Showing only tuples.
  postgres=# \a
  Output format is unaligned.
  postgres=# \f ' '
  Field separator is " ".
  postgres=# select * from example;
  1 1
  2 2
  3 3
  4 4
  postgres=# \o | /usr/bin/gnuplot
  postgres=# select 'set title "My Graph"; set terminal dumb 78 24; set key off; set ylabel "Time"; set xlabel "Servers";' || 'plot ''-'' with lines;' ; select * from example;
  postgres=# \o

                                    My Graph
  Time
      4 ++----------+----------+-----------+----------+-----------+---------**
        +           +          +           +          +            +     **** +
      |                                                           ****     |
  3.5 ++                                                      ****        ++
      |                                                   ****             |
      |                                               ****                 |
    3 ++                                           ***                    ++
      |                                        ****                        |
      |                                    ****                            |
  2.5 ++                               ****                               ++
      |                            ****                                    |
      |                        ****                                        |
    2 ++                    ***                                           ++
      |                 ****                                               |
      |             ****                                                   |
  1.5 ++        ****                                                      ++
      |     ****                                                           |
      + ****      +          +           +          +           +          +
    1 **----------+----------+-----------+----------+-----------+---------++
      1          1.5         2          2.5         3          3.5         4
                                     Servers

  postgres=#