title: ggplot_line from a big dataframe with different columns and variables
body: <p>I have a big dataframe with a lot of rows (+50k). There are some columns but the importants ones are "Date", "Stock", "Units_Sells". The dataframe is something like:</p>
  
  <pre><code>df &lt;- data.frame(Product=sample(LETTERS[1:4],15, replace=TRUE), 
                                Stock=sample(1:15,15,replace=TRUE), 
                                Store=sample(2340:2342,15,replace=TRUE),
                                Vta=sample(1:14,15,replace=TRUE),
                                Date=sample(seq(as.Date('1999/01/01'), as.Date('2000/01/01'), by="day"), 15))
  </code></pre>
  
  <p>And I would like to create a graph line store by store and Product when x-axis are the Date and y-axis are Units because the values (lines) should be Stock and Vta.
  I mean, I would like to do a graph like this for each Store-Product:
  <a href="https://i.stack.imgur.com/oePyq.png" rel="nofollow noreferrer">LINK TO THE GRAPH EXAMPLE</a></p>
  
  <p>I mean, one graph for each store&amp;product. I don't know if I'm explaining because english is not my native language.</p>
  
  <p>Thank you,</p>