Portable syntax for "set xtics rotate"

Gnuplot version 4.4 renders text within the drawing area with "set
xtics rotate angle 90". According to

http://newsgroups.derkeiler.com/Archive/Comp/comp.graphics.apps.gnuplot/2010-08/msg00079.html
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=586513

this is a feature, not a bug, but "set xtics rotate" give the expected
behavior everywhere (text from down to top, below the graph).

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>

gitstats

 set xdata time
 set timefmt "%Y-%m"
 set format x "%Y-%m"
-set xtics rotate by 90 15768000
+set xtics rotate
 set bmargin 5
 set grid y
 set ylabel "Commits"
 """
 set output 'commits_by_year.png'
 unset key
-set xtics 1 rotate by 90
+set xtics 1 rotate
 set grid y
 set ylabel "Commits"
 set yrange [0:]
 set format x "%Y-%m-%d"
 set grid y
 set ylabel "Files"
-set xtics rotate by 90
+set xtics rotate
 set ytics autofreq
 set bmargin 6
 plot 'files_by_date.dat' using 1:2 w steps
 set format x "%Y-%m-%d"
 set grid y
 set ylabel "Lines"
-set xtics rotate by 90
+set xtics rotate
 set bmargin 6
 plot 'lines_of_code.dat' using 1:2 w lines
 """)