/usr/lib/x86_64-linux-gnu/pkgconfig/plplotd-c++.pc is in libplplot-dev 5.10.0+dfsg2-0.1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | libdir=/usr/lib/x86_64-linux-gnu
includedir=/usr/include/plplot
drvdir=/usr/lib/x86_64-linux-gnu/plplot5.10.0/driversd
Name: PLplot C++
Description: Scientific plotting library (C++ bindings, double precision)
Requires.private: plplotd
Version: 5.10.0
Libs: -L${libdir} -lplplotcxxd
Cflags: -I${includedir}
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