Why bad linking of dots with Gnuplot?

I can't put all my data due to : It looks like your post is mostly code; please add some more details.

So you can find my data here : https://gitlab.com/v_alexandre/data/-/blob/master/data

script :

plot 'data' u 1:2 w l

As you can see I have two points linked but I don't understand why ?

Have you any ideas ?

Insert an emtpy line after line 6 of the data. This will disconnect the two islands.

Script:

### disconnect lines
reset session

$Data <<EOD
9.2710321280228 41.364959299521
9.265698545799 41.362164776515
9.2558829669799 41.368794222327
9.2602446048541 41.373651229899
9.273659440522 41.369221791575
9.2710321280228 41.364959299521

8.5734085639674 42.381404942274
8.5869206603959 42.378959456159
8.6000271534091 42.369259196001
8.6100160917347 42.373593854505
8.620173521868 42.372270719131
8.6408019259839 42.37343904341
8.6504088344609 42.362352538519
8.6596703887356 42.359406967225
8.6819749384239 42.358462008729
8.6841460127953 42.347988396554
8.7110680867563 42.347099989534
# ... not all data shown...
EOD

plot $Data u 1:2 w l
### end of script

Result:

Addition:

Just for the records, some more explanations and code which might be useful to others in similar cases. Your data seems to describe points of closed areas, e.g. coordinates of some islands. For some (unfortunate) reason your individual islands are not separated by empty lines, that's why gnuplot plots it in one go. How should gnuplot know where to separate? You have to tell gnuplot where to make a discontinuity. But how do you know?

Well, if these are all closed areas there is a start point and an end point which should be identical. So, you need to go through the data and everytime you find an identical datapoint you introduce a gap (empty line) afterwards. Of course, you don't want to do this manually, e.g. if you think of the Canadian islands or the lakes of Finland. Of course, you can do this with external tools and save your data to another file, but you could also do this with gnuplot. However, for the final result it might be sufficient to just "raise the pen" without modifying the data. With gnuplot you can play the "trick" and making this line transparent, e.g. color 0xff123456 (according to the scheme 0xAARRGGBB) it doesn't matter which color as long as it starts with 0xff, i.e. fully transparent.

In summary: define a function which sets the color to transparent everytime after a specific coordinate has appeared again. Maybe somebody can even simplify the function.

Data: SO65655925.dat

# x       y
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-72.8515  20.0937

Script: (works for gnuplot>=5.0.0, Jan. 2015)

### autoseparation of data describing closed areas 
reset session

FILE = "SO65655925.dat"

# function to set color to transparent when coordinate appears again
myColor(colX,colY,color) = \
    ($0==0 ? (x2=y2=NaN, c=0, x0=column(colX), y0=column(colY)) : 0, \
     x1=x2, x2=column(colX), y1=y2, y2=column(colY), \
     x1==x0 && y1==y0 ? c=c+1 : 0, c==2 ? (c=0,x0=x2,y0=y2) : 0, \
     c==0 && $0!=0 ? 0xff123456 : color )

set multiplot layout 2,1
    plot FILE u 1:2 w l lc "black" ti "Data as is"

    plot FILE u 1:2:(myColor(1,2,0xff0000)) w l lc rgb var ti 'Data with "autoseparation"'
unset multiplot
### end of script

Result: