title: gnuplot all points y value undefined log
body: <p>I would like to plot the log of my first and second column of my file "hmax.txt".
  But I get this error message and I don't understand why ?
  I haven't any 0 in my second column.</p>
  
  <pre><code>plot 'hmax.txt' u (log($1)):(log($2))
                                       ^
  "plot", line 18: all points y value undefined!
  </code></pre>
  
  <p>my plot file :</p>
  
  <pre><code>set term png size 800,800
  set output "lois.png"
  set style line 1 lc rgb "blue" lw 0.5
  set title 'hmax en fonction de t' font ",20"
  set xlabel 'temps t'
  set ylabel 'hmax'
  plot 'hmax.txt' u 1:2 with lp ls 1
  
  unset key
  set term png size 800,800
  set output 'loiss.png'
  set style line 1 lc rgb "blue" lw 0.5
  set title 'log(hmax) en fonction de log(t)' font ",20"
  set xlabel 'log(t)'
  set ylabel 'log(hmax)'
  set logscale xy
  plot 'hmax.txt' u (log($1)):(log($2)) with lp ls 1
  </code></pre>
  
  <p>And here my data file "hmax.txt" 
  <a href="https://drive.google.com/file/d/1ciutnt32knBRhBIjCcR6aaFfa8NxBRuM/view?usp=sharing" rel="nofollow noreferrer">https://drive.google.com/file/d/1ciutnt32knBRhBIjCcR6aaFfa8NxBRuM/view?usp=sharing</a></p>
  
  <p>Thank you very much !</p>