gnuplot: how to add an horizontal line from the last point plotted

Question

I have some time-based values that cycle between 10000 and 0 (about twice a day).
I've drawn the graph but I'd like an horizontal line from the last point for easy reference.

My data (repeats on either column are expected):

-23.7897222222222 4832
-23.7138888888889 4749
-23.7138888888889 4749
-23.5169444444444 4479
-23.4277777777778 4365
-23.4152777777778 4365
-23.2647222222222 4086
-23.1686111111111 3960
-23.1083333333333 3907
-23.1083333333333 3907
 ...
-0.777777777777778 4950
-0.777777777777778 4950
-0.6075 4601
-0.6075 4601
-0.421666666666667 4407
-0.334722222222222 4213
-0.284722222222222 4213
-0.284722222222222 4213
-0.112222222222222 3998
-0.112222222222222 3998

My gnuplot script:

set terminal png
set title 'generated on 2020-10-06 16:00:00'
set xrange [-24:0]
set xtics -24, 4
set ytics 0, 2500
set output 'xxx.png'
set arrow from graph 0.042,0 to graph 0.042,1 nohead
set arrow from graph 0.083,0 to graph 0.083,1 nohead
set arrow from graph 0.125,0 to graph 0.125,1 nohead
set arrow from graph 0.167,0 to graph 0.167,1 nohead
set arrow from graph 0.208,0 to graph 0.208,1 nohead
set arrow from graph 0.250,0 to graph 0.250,1 nohead
set arrow from graph 0.292,0 to graph 0.292,1 nohead
set arrow from graph 0.333,0 to graph 0.333,1 nohead
set arrow from graph 0.375,0 to graph 0.375,1 nohead
set arrow from graph 0.417,0 to graph 0.417,1 nohead
set arrow from graph 0.458,0 to graph 0.458,1 nohead
set arrow from graph 0.500,0 to graph 0.500,1 nohead
set arrow from graph 0.542,0 to graph 0.542,1 nohead
set arrow from graph 0.583,0 to graph 0.583,1 nohead
set arrow from graph 0.625,0 to graph 0.625,1 nohead
set arrow from graph 0.667,0 to graph 0.667,1 nohead
set arrow from graph 0.708,0 to graph 0.708,1 nohead
set arrow from graph 0.750,0 to graph 0.750,1 nohead
set arrow from graph 0.792,0 to graph 0.792,1 nohead
set arrow from graph 0.834,0 to graph 0.834,1 nohead
set arrow from graph 0.875,0 to graph 0.875,1 nohead
set arrow from graph 0.917,0 to graph 0.917,1 nohead
set arrow from graph 0.958,0 to graph 0.958,1 nohead
plot 'xxx.dat' notitle with points pt 3

And my (intended) result

Answer 1

Check the following. The value myLast will contain the last y-value of the data. Plotting your grid can be done simpler.

Code:

### horizontal line through last datapoint
reset session

$Data <<EOD
-23.7897222222222 4832
-23.7138888888889 4749
-23.7138888888889 4749
-23.5169444444444 4479
-23.4277777777778 4365
-23.4152777777778 4365
-23.2647222222222 4086
-23.1686111111111 3960
-23.1083333333333 3907
-23.1083333333333 3907
 ...
-0.777777777777778 4950
-0.777777777777778 4950
-0.6075 4601
-0.6075 4601
-0.421666666666667 4407
-0.334722222222222 4213
-0.284722222222222 4213
-0.284722222222222 4213
-0.112222222222222 3998
-0.112222222222222 3998
EOD

set title 'generated on 2020-10-06 16:00:00'

set xrange [-24:0]
set xtics -24, 4
set mxtics 4
set grid xtics,mxtics ls -1, ls -1
set yrange [0:10000]
set ytics 0, 2500

plot $Data u 1:(myLast=$2) w p pt 3 notitle, \
     myLast w l notitle
### end of code

Result: