title: Plotting implicit function
body: <p>I'm trying to plot the following implicit formula in R:</p>
  
  <p>1 = x^2 + 4*(y^2) + x*y</p>
  
  <p>which should be an ellipse. I'd like to randomly sample the x values and then generate the graph based on those. </p>
  
  <p><a href="https://stackoverflow.com/questions/5285178/r-plot-implicit-function-outer-command">Here's</a> a related thread, but the solutions there seem to be specific to the 3D case. This question has been more resistant to Googling that I would have expected, so maybe the R language calls implicit formulas something else.</p>
  
  <p>Thanks in advance!</p>