gnuplot development

Running the following gnuplot v4.2.6 commands on Mac OS X 10.5.8. Otherwise
this installation of gnuplot works perfectly for quite some time.

$ gnuplot
set terminal png nocrop enhanced font Verdana 8 size 400,300
set title "Fixed By Priority" font "Verdana-Bold,10"
set key outside
set xlabel "Time"
set ylabel "Bug Count"
set xdata time
set xtic rotate by -75
set timefmt "%Y-%m-%d"
set format x "%m-%d-%y"
set style data linespoints
set output "Bugzilla_FixedByPriority.png"
plot "Bugzilla_FixedPri1.dat" using 1:3 title 2, \
"Bugzilla_FixedPri2.dat\" using 1:3 title 2, \
"Bugzilla_FixedPri3.dat\" using 1:3 title 2, \
"Bugzilla_FixedPri4.dat\" using 1:3 title 2, \
"Bugzilla_FixedPri5.dat\" using 1:3 title 2, \
"Bugzilla_Fixed.dat\" using 1:2 title 'Total'

Using these data files:

Bugzilla_FixedPri1.dat:
2009-02-13 P1 1
2009-02-16 P1 1
2009-02-19 P1 2
2009-03-03 P1 1
2009-06-29 P1 2
2009-06-30 P1 1

Bugzilla_FixedPri2.dat:
2009-02-19 P2 1
2009-02-20 P2 1
2009-06-26 P2 2
2009-06-29 P2 1

Bugzilla_FixedPri3.dat:
2009-02-20 P3 1
2009-06-26 P3 1

Bugzilla_FixedPri4.dat:
2009-02-18 P4 3
2009-02-20 P4 1
2009-06-26 P4 1

Bugzilla_FixedPri5.dat: (empty file)

Bugzilla_Fixed.dat:
2009-02-13 1
2009-02-16 1
2009-02-18 3
2009-02-19 3
2009-02-20 3
2009-03-03 1
2009-06-26 4
2009-06-29 3
2009-06-30 1

Produces a warning about missing data for Bugzilla_FixedPri5.dat, which is
fine, but it also produces a bad plot image that I've attached to this bug.
If I change the plot command parameters for the missing data from:
"Bugzilla_FixedPri5.dat\" using 1:3 title 2, \
to:
"Bugzilla_FixedPri5.dat\" using 1:3 title 'P5', \
the plot appears correctly. This is also attached.

Somehow the parameter to use a title for the Pri5 line using a column from
the input data file interferes with the graph generation, even though the
missing data should cause it to not affect the generated plot. Using a
static title string fixes the problem, but this plot must get it's labels
from the data file.