How to plot multiple colors according to column value and display a legend?

I have the following sample data in a file ("gnuplot_data"):

117 183 C3_V1
74  164 C3_V2
162 53  C3_V3

I want to make a scatter plot where the first 2 columns are X,Y coordinates and the value in column 3 is used to apply a specific color to all points matching that value. I also want to include a legend (not a color palette) that shows which values map to which color of point, e.g. a red square for C3_V1, blue for C3_V2, and green for C3_V3. I would also prefer to not use external programs such as awk. Is it possible to do this entirely within gnuplot? Based on my searching I've come up with the following:

set linetype 1 linecolor 'green' pt 5
set linetype 2 linecolor 'red' pt 5
set linetype 3 linecolor 'blue' pt 5
set linetype 4 linecolor 'black' pt 5
set key right top
set xrange [0:200]
set yrange [0:200]
plot "gnuplot_data.txt" using 1:2:(stringcolumn(3) eq "C3_V1" ? 1 : stringcolumn(3) eq "C3_V2" ? 2 : stringcolumn(3) eq "C3_V3" ? 3 : 4) with points lc variable notitle
pause -1

The plot does what I want except there is no legend despite the set key right top command:

Why is the legend not displaying? Do I need to take a fundamentally different approach to create this plot?

Solution

If you are plotting one dataset (although with variable colors) you will get one legend entry (keyentry). However, as I understand you want as many keyentries as you have different datapoint types/colors. I assume that you don't want to change your original datafiles by adding an extra color column.

My suggestion would be:

add a datablock to provide a lookup table for your type and the corresponding color
(mis)use the sum() function to create a lookup function
a customizable default color is added for unspecified entries

Depending on your data and requirements you could maybe even simplify the following script.

Script:

### plot with variable color and keyentries for each color
reset session

$Data <<EOD
117  183  C3_V1
 74  164  C3_V2
162   53  C3_V3
100  100  ???
EOD

$Colors <<EOD
C3_V1   0xff0000
C3_V2   0x0000ff
C3_V3   0x00ff00
other   0xcccccc   # last entry is default color and legend entry
EOD

set xrange [0:200]
set yrange [0:200]
set key right top noautotitle

color(i)         = int(word($Colors[i],2))
type(i)          = word($Colors[i],1)
lookupColor(col) = (c=color(|$Colors|), sum[i=1:|$Colors|] \
                   (type(i) eq strcol(col) ? c=color(i) : 0), c)

plot $Data u 1:2:(lookupColor(3)) w p pt 5 ps 2 lc rgb var, \
     for [i=1:|$Colors|] keyentry w p pt 5 ps 2 lc rgb color(i) ti type(i) noenhanced
### end of script

Result:

If the categories/types appear only once in your data and if you are willing to add a color column you can get the same result with the following script:

Script 2: (output graph same as above)

### plot with variable color and keyentries for each color
reset session

$Data <<EOD
117  183  C3_V1  0xff0000
 74  164  C3_V2  0x0000ff
162   53  C3_V3  0x00ff00
100  100  other  0xcccccc
EOD

set xrange [0:200]
set yrange [0:200]
set key right top noautotitle

stats $Data u (rows=column(0)) nooutput  # get max. row index

plot $Data u 1:2:4 w p pt 5 ps 2 lc rgb var, \
     for [r=0:rows] $Data every ::r::r u 1:(t=strcol(3),NaN):4 w p pt 5 ps 2 lc rgb var ti t noenhanced
### end of script