title: Make plot from 3 columns in R
body: <p>I have 3 columns in a dataframe, it looks like this :</p>
  
  <pre><code>member_id  quantity  time
  a1         2         16-jan-2016
  b2         5         16-jan-2016
  a1         1         17-jan-2016
  b2         10        19-jan-2016
  </code></pre>
  
  <p>and I want to make a graph/plot like this :</p>
  
  <p><a href="https://i.stack.imgur.com/f1ogI.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/f1ogI.jpg" alt="enter image description here"></a></p>
  
  <p>With x-axis is <strong>time</strong><br>
  y-axis is <strong>quantity</strong> <br>
  and each line represents a <strong>member_id</strong><br></p>
  
  <p>so far, I use this code : <br></p>
  
  <pre><code>ggplot(df,aes(time,quantity))
  </code></pre>
  
  <p>but it doesn't turn out like I want.<br>
  So how can I solve this problem?</p>