Which module can I use to visualize Drupal log data in the format of a chart?
Native charting modules

Have a look at the Comparison of Charting modules. Here is a quote from it:


  There are a variety of modules that facilitate the creation of charts. When it comes to charting in Drupal, the saying "There's a module for it" doesn't really apply. In this case, it's more accurate to say, "There are plenty of modules for it" ...
  
  These modules support for various charting libraries that may be desirable based on a site's desired appearance, feature-set, and licensing needs.


Here is the shortlist of 'native' charting modules, for which at least 1 official release exist (quote from that comparison also):


Forena facilitates the creation of charts  by using the SVG Graph library. Forena does not depend on Views for creating charts, however it can be used as an extra views style plugin.
Google Chart Tools provides a simple API to build interactive SVG charts, using the Google Chart Tools.
Chart currently only supports the deprecated Google Chart Tools: Image Charts, which is deprecated as of April 20, 2015. Though issues #2371567, #2371075 and #2368793 are about addressing that. It is the most commonly used charting module today (over 25K reported installs, which is more then double of all other native charting modules together).
Charts (not to be confused with the charting module with namespace chart !!!) provides an API that can be used as a stand-alone API, and the integration with Views supports building charts with no coding.
Charts and Graphs, which is an API for developers. It can be extended by third-party modules that want to add their own charting implementations. It does nothing by itself. It should only be installed if some other module requires it. Views Charts is one of the modules using it.
Flot provides an API and views integration for using the Flot graphing library.


Note: there are quite a few additional 'native' charting modules, though they do not have an official release (yet), only dev, alfa or beta versions.

Forena showcase

Have a look at my answer to "Views Table with Taxonomy Terms for Columns and Rows", which details how I once created a crosstab report (using the Forena module) that looked like so:



Then look at the video demo "How to create Crosstab Tables and (related) charts" to get an idea what it takes to create a chart from such crosstab tables.

Disclosure: I'm the author of the comparison and co-maintainer of the Charts, Chart and Forena.