title: gnuplot: same grid intervals, round lable values, draw lines
body: <p>few questions on gnuplot:</p>
  <ol>
  <li>How to set the same intervals of grid tics (same grid step) both in value and in pixels on both axes?  Tics interval is set manually, but autoscale method is wanted (if different initial data has different max values) with condition that step and min-max will be the same for x and y. If it possible in gnuplot. Maybe request is very special.</li>
  <li>How to round label values? (if data is 1.5000 , label is 1.5000, but want: 1  1.5  2  2.5 etc, floor($1) and int($1) doesn't work?).</li>
  <li>Is it possible to insert a line connecting points with the max distance of 0,0 (as in the example picture) (maybe there should be a condition, if x^2+y^2 = max, then draw a line)</li>
  <li>How to highlight with a bold line those grid lines that intersect 0,0? Or draw separate lines (done, but maybe better way exists)?</li>
  </ol>
  <p>Example:</p>
  <p>[<img src="https://i.stack.imgur.com/JIuWx.png" alt="example" /><a href="https://i.stack.imgur.com/JIuWx.png" rel="nofollow noreferrer">1</a></p>
  <p>My graph:</p>
  <p><a href="https://i.stack.imgur.com/zEcVx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zEcVx.png" alt="my graph" /></a></p>
  <pre><code>set terminal windows 0 position 20,20
  set terminal windows size 800,800
  
  set xtics 0.25
  set ytics 0.25
  
  set grid xtics ytics
  
  set size ratio -1 # now length of axes on monitor are ~ 17.5 cm and 18.5 cm, not square
  #set size square
  
  set style circle radius graph 0.25/2
  
  maxv=1
  set yrange [-maxv:maxv]
  set xrange [-maxv:maxv]
  
  set ylabel 'Y' font &quot;, 11&quot;
  set xlabel 'X' font &quot;, 11&quot;
  
  filename='file1.txt'
  
  set arrow from 0, graph 0 to 0, graph 1 lc 'gray' nohead
  
  
  plot 0 lc 'gray' notitle, \
  filename using ($2-$3):($4-$5):(floor($1)) \
  with labels tc lt 1 point pt 7 offset char 1,1 notitle, \
  filename using ($2-$3):($4-$5) w l lc black notitle
  
  </code></pre>
  <p>version 5.2</p>
  <p>data:</p>
  <pre><code>-6.0    -10.14  -10.10  14.92   14.86
  -5.5    -16.75  -16.70  25.60   25.50
  -5.0    -20.62  -20.53  30.65   30.49
  -4.5    -26.56  -26.42  38.00   37.74
  -4.0    -27.80  -27.55  38.25   37.84
  -3.5    -33.33  -32.94  43.80   43.19
  -3.0    -33.96  -33.37  44.04   43.19
  -2.5    -38.02  -37.20  46.64   45.85
  -2.0    -38.97  -38.07  44.53   44.18
  -1.5    -30.61  -29.97  34.93   35.03
  -1.0    -22.60  -22.00  24.24   24.22
  -0.5    -7.65   -7.25   5.70    5.47
  0.0      2.87    2.84    -3.67   -3.66
  0.5      17.62   17.21   -19.79  -19.57
  1.0      17.37   16.77   -17.00  -16.95
  1.5      30.11   29.50   -32.04  -32.20
  2.0      34.69   33.99   -40.26  -40.04
  2.5      38.21   37.44   -45.19  -44.37
  3.0      34.26   33.67   -42.68  -41.83
  3.5      26.69   26.31   -34.85  -34.24
  4.0      30.04   29.79   -41.45  -41.03
  4.5      20.62   20.48   -28.99  -28.74
  </code></pre>