title: Read complex numbers from a single column in gnuplot
body: <p>I'd like to plot data which is stored on a file in three columns like this :</p>
  
  <pre><code>316   0.662500024   180.382+137.860i
  317   0.664285719   188.515+135.229i
  318   0.666071415   196.834+131.997i
  319   0.667857111   205.536+127.380i
  320   0.669642866   213.829+122.105i
  321   0.671428561   222.220+116.453i
  </code></pre>
  
  <p>The first column contains the sample's number, so it's not really interesting for me. The second column contains a frequency, and the third column contains a complex impedance measured with a VNA.
  What I would like to do is split the real and complex parts to plot them separately, or plot the complex number magnitude.</p>
  
  <p>I tried something like this, according from what I read online :
  <code>plot 'datafile.data' using (1000*$2):(sqrt($3**2+$4**2)) '%lf  %lf%lfi'</code>*
  But it doesnt work, which is not surprising since I did this in the blind...</p>
  
  <p>Thank you for your help</p>