Custom gnuplot axis label for time series

The subject whose data I want to display was born 1988-04-20.

I would like to have the X2 axis display the integral ages of the subject thus:

   | x2 label | Corresponding x1 value |
   |----------+------------------------|
   | "18"     | 2006-04-20             |
   | "19"     | 2007-04-20             |
   | "20"     | 2008-04-20             |
   | "21"     | 2009-04-20             |

Script:

reset session;

$Data <<EOD
2006-03-24 150
2007-01-12 147
2007-11-12 149
2007-11-15 142
2008-03-29 166
2008-12-16 173
2009-05-06 170
EOD

birth_date  = "1988-04-20";

set xdata   time;
set timefmt "%Y-%m-%d";
set xtics   format "%Y-\n%m";
set x2label "Age";
set ylabel  "Weight (lbs.)";

plot $Data using 1:2;

Result:

There are two ways to plot time data, where I usually prefer the second way.

set xdata time
set timefmt "%Y-%m-%d"
set format x "%Y\n%m"  
plot $Data u 1:2

or

set format x "%Y\n%m" timedate
plot $Data u (timecolumn(1,"%Y-%m-%d")):2

Furthermore, you can link x and x2 axis, check help link. I guess this might be easier with the second way (actually, haven't tried the first way).

For further reading, check help time/date, help timecolumn and help strptime.

Script:

### plotting date on x1 and age on x2
reset session

$Data <<EOD
2006-03-24  150
2007-01-12  147
2007-11-12  149
2007-11-15  142
2008-03-29  166
2008-12-16  173
2009-05-06  170
EOD

myFmt       = "%Y-%m-%d"
SecsPerYear = 3600*24*365.25
birth_date  = strptime(myFmt, "1988-04-20")

set format x "%Y\n%m" timedate
set xtics nomirror
set x2tics numeric 0.5 nomirror
set x2label "Age"
set ylabel "Weight (lbs.)"
set grid x,y
set key noautotitle
set link x2 via (x-birth_date)/SecsPerYear inverse (x*SecsPerYear+birth_date)

plot $Data u (timecolumn(1,myFmt)):2 w lp pt 7
### end of script

Result: