title: 'x' and 'y' lengths differ ERROR when plotting
body: <p>I just started using R.
  I am supposed to Calculate a new variable “Vehic_vol” from the sum of “Psgr_Vol” and “Lugg_Vol” and Plot this new variable against “CITY_MPG” for the whole data set but I end up with 'x' and 'y' lengths differ ERROR! Any thoughts?</p>
  <p>Here is what I did: <br></p>
  <pre class="lang-r prettyprint-override"><code>Vehic_vol&lt;-(VehicleData$Psgr_Vol + VehicleData$Lugg_Vol)
   plot (VehicleData$Vehic_vol, VehicleData$CITY_MPG)
  </code></pre>
  <pre><code>Error in xy.coords(x, y, xlabel, ylabel, log) : 
    'x' and 'y' lengths differ
  </code></pre>