如何隐藏gnuplot轴但仍显示网格？

Question

看到从中心向右延伸的黑线（描绘r轴）？如何隐藏它，但仍显示（极坐标）网格？

这是gnuplot语法：

set label '#4' right at screen 0.86, screen 0.88 font 'Helvetica-Bold,34'
set polar
set grid polar back
set xtics nomirror
set ytics nomirror
set rtics scale 0 format ""
set xlabel "Metrology X axis (mm)"
set ylabel "Metrology Y axis (mm)"
set size square
set border 0
set encoding iso_8859_1
set title "80Hz, \26110.00V, mean step size: 23.8\2610.9 um        15 Mar 2017 22:06PM" 
set key outside right
plot '-' using 2:($1/1000.0) t '20', '-' using 2:($1/1000.0) t '50'

我在Linux上使用Gnuplot v5.0.5。