gnuplot: simple beeswarm example

I have been struggling with a basic beeswarm plot from page 62 in this doc. I imagine they are skipping some details, and I'm not sure what actual data they used. I think in particular the problem is mapping a categorical/string variable to an X-axis value.

I used this data:

A 1
A 2
A 3
B 4
B 5
B 6

With this script:

set terminal png
set output "graph.png"
set jitter
plot "data.csv" using 1:2:1 with points lc variable

I get this error:

"graph_script" line 4: warning: Skipping data file with no valid points

plot "data.csv" using 1:2:1 with points lc variable
                                                   ^
"graph_script" line 4: x range is invalid

In their demos gallery, I see something like set xtics ("A" -1, "B" 0) which could maybe help me to label already-numeric data better, but what if my data doesn't start off numeric to begin with?

Do I need something like (hash_string_to_large_int($1) % 2)? There must be an easier way!

Solution

As mentioned in the comments you have to "convert" your keys into numbers in order to plot them. You can do this by creating a list with your unique keywords and defining a function to get the indices.

First, the following example creates some random data
The code after knows nothing about the keywords, so it creates the unique list from scratch from the random data.

Maybe there is (and I am not aware) a simpler solution with gnuplot only.

Code:

### bee-swarm plot with string keys
reset session

# create some random test data
myExts = '.py .sh .html'
set print $Data
    do for [i=1:100] {
        print sprintf("%s %d",word(myExts,int(rand(0)*3)+1),int(rand(0)*10+1)*5)
    }
set print

# create a unique list of strings from a data stringcolumn
Uniques = ''
addToList(list,col) = list.( strstrt(list,'"'.strcol(col).'"') > 0 ? '' : ' "'.strcol(col).'"')
stats $Data u (Uniques = addToList(Uniques,1),0) nooutput

getIdx(key) = (_idx=NaN, sum [_i=1:words(Uniques)] (word(Uniques,_i) eq key ? _idx=_i : 0), _idx)

set offsets 0.5,0.5,1,1
set key noautotitle

set multiplot layout 1,2

    set title "No jitter"
    plot $Data u (idx=getIdx(strcol(1))):2:(idx):xtic(word(Uniques,idx)) w points pt 7 lc var

    set title "With jitter"
    set jitter
    replot
unset multiplot
### end of code

Result: