https 2d contour

https://zhuanlan.zhihu.com/p/363091144

http://www.phyast.pitt.edu/~zov1/gnuplot/html/contour.html 最好

https://www.codenong.com/19406639/

https://zhuanlan.zhihu.com/p/356438078

http://gnuplot.sourceforge.net/demo/contours.html