Count of users who are logged in per day I searched to find a way to display the number of users who are logged in per day.  

I want to display a chart using views for last month to show the login statistics of the website.

Any suggestion?
Well in other to achieve this, you will be using a combination of modules. 

Use the counter module for gathering that kind of statistics. Another option is to enable the statistics module in Drupal core


  Counter module counts how many visitors on your website. This module
  provides real time counting. All data saved to database.


And with google chart tools and the views module create a chart


  Display live data on your site using Google Chart Tools Google Chart
  Tools module provides a simple API to Build custom interactive SVG
  charts. The module contain an example module which demonstrate how
  this module can be use.