gnuplot: use 'w histeps' for files_by_date.

'smooth csplines' showed negative file count sometimes, which wasn't nice ;-)

gitstats

@@ -816,7 +816,7 @@ set format x "%Y-%m-%d"
 set ylabel "Files"
 set xtics rotate by 90
 set bmargin 6
-plot 'files_by_date.dat' using 1:2 smooth csplines
+plot 'files_by_date.dat' using 1:2 w histeps
 """)
 		f.close()