Gnuplot: conditional plotting ($2 == 15 ? $2 : '1/0') with lines

Question

My data looks like this:

10:15:8:6.06000000:
10:15:2:19.03400000:
10:20:8:63.50600000:
10:20:2:24.71800000:
10:25:8:33.26200000:
10:30:8:508.23400000:
20:15:8:60.         


        

Answer 1

...just stumbled across this old question... Well, it's not "acceptable" that you need an external tool for such a basic task when you want to plot the filtered data connected with lines or with linespoints. There is a gnuplot-native solution. The "trick" of the workaround is to plot several data points on top of each other and only change the coordinates if a new point has been found. The code is as simple as this:

### conditional plot with connected lines or linespoints
reset session

# added two datapoints for testing purposes
$Data <

Result:

Addition:

just for completeness. Actually, set datafile missing "NaN" is solving the problem in gnuplot5.x, but since this question was from gnuplot4.6 times... and some people seem to still plot with version 4.x

SO_Filter.dat

# added two datapoints for testing purposes
10:15:8:6.06000000:
10:15:2:19.03400000:
10:20:8:63.50600000:
10:20:2:24.71800000:
10:25:8:33.26200000:
10:30:8:508.23400000:
13:20:8:8.88888888:
15:15:8:9.99999999:
20:15:8:60.06300000:
20:15:2:278.63100000:
20:20:8:561.18000000:
20:20:2:215.46600000:
20:25:8:793.36000000:
20:25:2:2347.52900000:
20:30:8:5124.98700000:
20:30:2:447.41000000:

The code:

### conditional plot with connected lines or linespoints
reset
FILE = "SO_Filter.dat"
set datafile separator ":"

set multiplot layout 2,1  title "generated with gnuplot 4.6"

# this works with gnuplot 4.x and 5.x
x0 = y0 = NaN
plot FILE u ($2==15 && $3==8 ? (y0=$4,x0=$1) : x0):(y0) w lp pt 7 ti "works with gnuplot >4.x and 5.x"

# this works with gnuplot >5.x
set datafile missing "NaN"
plot FILE u ($2==15 && $3==8 ? $1 : NaN ):4 w lp pt 7 ti "works only with gnuplot >5.x"

unset multiplot
### end of code

Result in gnuplot 4.6: