Log Message:
Update to gnuplot 4.2.6.

New features, changes and fixes in gnuplot version 4.2.6
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* NEW xterm tektronix emulation 'set term xterm'
* FIX 'set xtics mirror' didn't work if xtics were previously unset
* FIX off-by-one pixel bug in width of boxes with palette or rgb color
* FIX center rotation of 'set view equal xyz' mode at screen center
* FIX sanity-check time ranges for axes with timeformat
* FIX pslatex blacktext and broken format specifier
* FIX PostScript code points for Lcaron, lcaron in encoding cp1250
* CHANGE If a 2D plot uses a Z-based palette, then autoscale cbrange
* CHANGE aquaterm accepts "size xx,yy" with a comma
* CHANGE Remove the EXPERIMENTAL flag from the wxt terminal
* CHANGE Remove the EXPERIMENTAL flag from the x11 terminal binary polygon mode