title: Horizontal grid lines in plotly R
body: <p>The standard R plotly scatter/line graph comes with both vertical and horizontal grid lines. I cannot find the command to select only vertical or horizontal lines. I want the latter. So far I have found the following commands in the <a href="https://plot.ly/r/axes/" rel="noreferrer">offiial guide</a>:</p>
  
  <pre><code>showgrid = FALSE/TRUE
  gridcolor = ...
  gridwidth = ...
  </code></pre>
  
  <p>Any idea which is the command to select subset of grid lines?</p>