Polar mode improvements and extensions

• Polar coordinates may be used in label, arrow, and object definitions
• set [m]ttics places ticmarks and labels on the perimeter of a polar plot. See http://gnuplot.info/demo_5.4/ttics.htmlpolar axis and ticlabels demo

• set rlabel (p. ) places a label above the r axis
• Inverted rrange (p. ) (i.e. set rrange [90:0]) allows use of celestial horizontal coordinates. See http://gnuplot.info/demo_5.4/solar_path.htmlsolar path demo

• set border polar (p. ) draws a solid line around the perimeter of a polar plot
• set theta (p. ) controls the position of theta = 0 around the perimeter of a polar plot and the sense (clockwise or anti-clockwise) of increasing theta