how to use gnuplot to draw colored points

Question

how can I use gnuplot to plot isolated points(not connected points): the x axis must be the first column which is the date, the y axis is the second columns the 3rd column must be the color of the point(colors are defined into ranges (ex: from 1 to 3 red, from 4 to 6 green...)

1999-01-19  21  0
2009-07-01  0   1
2008-08-20  2   1
2008-12-18  1   1
2004-05-12  4   1
2009-07-29  2   1
2008-08-07  0   1
2006-03-08  1   1
2004-08-31  9   2
2001-03-27  12  2
2009-08-19  0   2
2010-07-14  2   3
2009-06-24  4   3
2009-11-11  33  4
2010-10-13  5   4
2012-02-22  34  4
2011-05-11  2   5

Answer 1

There are several ways to do that:

You can define several line style and then use linecolor variable, which allows you to use the last column as line style index:

set timefmt '%Y-%m-%d'
set xdata time
set format x '%Y-%m'

set style increment user
set style line 1 lc rgb 'red'
set style line 2 lc rgb 'blue'
set style line 3 lc rgb 'green'
set style line 4 lc rgb 'magenta'
set style line 5 lc rgb 'yellow'

set style data points
plot 'data.txt' using 1:2:3 linecolor variable pt 7 ps 2 t ''

The result is:

This requires you to define as many line styles as you have colors.

Alternatively you can define a respective color palette and use linecolor palette to do the coloring of the points:

set timefmt '%Y-%m-%d'
set xdata time
set format x '%Y-%m'

set cbrange [1:6]
set palette defined (1 'red', 3.5 'red', 3.5 'green', 6 'green')

set style data points
unset colorbox
plot 'data.txt' using 1:2:3 linecolor palette pt 7 ps 2 t ''

When using this, you must pay attention, that the cbrange and the values used for the palette defined match (here 1 and 6), because in general the values of the palette are adapted to match the color range.

The above script gives the result:

Answer 2

Some solution could be:

plot for [i=0:5] 'cp.txt' using ( i==$3 ? $1 : 1/0 ):2 notitle with points linestyle i