Update gnuplot to 5.2.7:

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Release Notes date: 29-May-2019

Changes in 5.2.7
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* NEW "set pm3d noclipcb" will skip drawing quadrangles with color < cbmin
* NEW pm terminal supports utf8, dashed lines, bold/italic text
* NEW imaginary component of array values is available to "using" in column 3
* NEW gnuplot --slow may help use of qt terminal with slow system font server
* CHANGE splot "with points pt 0" acts like "with dots"
* CHANGE (tikz) improved default interpretation of gnuplot arrowhead style
* CHANGE teach hidden3d to handle double-headed vectors (arrowstyle "heads")
* CHANGE "unset title" or "reset" resets all title properties to default
* CHANGE font names passed to enhanced text processing may be in quotes
* CHANGE cairo terminals accept fractional font sizes
* CHANGE "plot with table" does not output trailing field separator
* CHANGE better centering for clustered histograms
* CHANGE clear STATS_* variables before performing analysis
* CHANGE cairo, libgd, and svg terminals handle LT_NODRAW by not drawing at all
* CHANGE svg terminal clickable icons are in-line data rather than external refs
* CHANGE revise space allocated for ylabel; allow text justification for ylabel
* CHANGE reduce overhead, making it feasible to have 10^6 hypertext labels
* FIX "plot with table" unnecessarily limited string column output to 60 chars
* FIX (windows) make sure graph window is shown even if the program is "hidden"
* FIX hidden3d mode was not correctly dealing with arrowstyle backhead
* FIX sprintf in 'using' spec must not clobber numeric locale
* FIX tikz - request for polygon with 0 vertices could hang terminal
* FIX lua/tikz - do not attempt to ftruncate output being piped through stdout
* FIX tkcanvas - do not attempt to ftruncate output being piped through stdout
* FIX handle NaN values in input stream to "stats FOO matrix"
* FIX prevent extra read past the end of a datablock holding matrix data
* FIX pm - issues in box and polygon fill, (wide) lines, enhanced text, images
* FIX dropped character in piped input stream if plot window is manually closed
* FIX extraneous dots and potential segfault from splot with contour labels
* FIX boxplots - color sequence correctly starts with specified linetype
* FIX incomplete initialization led to segfault from "gnuplot -c foo.gp baz"
* FIX Allow "pm3d depthorder base" to handle 3D quadrangles with logscale z

(rin)