New plotting style 'marks', 'linesmarks' and new object 'mark'

[本吉 弘岐 <him...@ic...>]

Hello,

I have created a ticket in "Patches" to add a new plotting style, 'marks'. This is a 2D plotting style that draws marks represented by user-defined polygons or polylines at specified coordinates. The 'linesmarks' style corresponding to linespoints and the 'mark' object for placing individual marks are also proposed in it. For more information, please see the proposal documentation attached to the "Patches" ticket page.

https://sourceforge.net/p/gnuplot/patches/815/

I would appreciate your input on its implementation.