How to combine 3 line charts in a single chart, using the Views integration of the Charts module?
After diving into deep on visualization with Drupal, Charts module gives us the opportunity to do this, either by using the Views integration of Charts (to create charts without coding PHP snippets), or by using the Charts API (used as a developer module that provides a unified API for the integration of various charting solutions into Drupal).

This is from the Readme.txt file, incuded with the Charts module:


  Creating Multiple Series and Combo Charts in the UI
  
  When using Views to build your charts, you may find it difficult to
  retrieve more than a single set of data generated by a COUNT() query.
  For example if you wanted to retrieve the age of all your site users,
  but display "Male" and "Female" values in a column chart at the same
  time, constructing the underlying table of data is quite difficult.
  
  To solve this problem, you can combine multiple charts on top of each
  other. The "parent" chart provides the global information, such as the
  height, width, title, and other properties. Charts that are "children"
  provide only data and (optionally) a secondary axis. After you've
  assembled the first series of data in your chart according to the
  instructions in the "Creating Charts in the UI" section, add a new
  display to the same view of the type "Chart Add-on". The "Chart
  Add-on" type is added the same way you would add a new Page or Block
  display, from the "+ Add" menu at the top of the view configuration.
  
  After this new display has been added, find the setting for "Combine
  with parent chart" and change this value to point at the parent chart
  you have already assembled. Then adjust the settings for the child
  chart to pull in different data (often by overriding the filter
  settings). Now you can go back to your parent display, and see that
  the results from the child chart have been merged into the results
  from the parent chart. You can even use this approach to combine
  different types of charts, such as a line chart over the top of a
  column chart. Note that not all chart types can be combined together
  and invalid combinations may cause your chart to throw errors.