title: plotting abline with multiple regression in R
body: <p>I am performing a multiple regression on 4 predictor variables and I am displaying them side-by-side... but I just cant figure out how to plot the abline whilst having them side-by-side. 
  Just for clarity, here's my code:</p>
  
  <pre><code>fit1 &lt;- lm(bband ~ ProfHH + Employed + TwoCars + HEQual, data = County)
  layout(matrix(c(1,2,3,4), 2, 2, byrow = TRUE))  ## display 4 plots side by side
  plot(bband ~ ProfHH +  Employed + TwoCars + HEQual, data = County)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ya14T.png" rel="nofollow noreferrer">Image of the 4 plots side-by-side</a></p>
  
  <p>The problem Im having is I don't know how to add the abline whislt plotting them this way!? 
  Normally I'd just use <code>abline(fit1)</code>but this is producing the following error:</p>
  
  <pre><code>"abline" is not a graphical parameter
  </code></pre>
  
  <p>Any suggestions as to how I'd fix this??</p>