How do I make a list of Users and their total number of posts in a given date?
You may want to check if using the Views Date Format SQL module could be a solution for you. Here are some quotes about it (from its project page):


  ... allows to format date fields using SQL.
  
  The core functionality is to remove the date formatting from render() and put it in query(). I.e. format date values using SQL's DATE_FORMAT rather than PHP's format_date.
  This is achieved by assigning a new default handler to node 'created' and 'changed' date fields. This handler extends and overrides views's build in views_handler_field_date.
  
  The UI is completely unobtrusive, only a single checkbox "Use SQL to format dates" is added to the handler configuration popup.


Your question is not about creating charts in Drupal, but you may want to refer to this comment also for an example of how we typically use it within the Charts (disclosure: I'm a co-maintainer) module.