IDL is the data analysis program that I use in my research. That is why it is referenced in some of the materials that are available for download. The majority of data plots (if not all of them) on this site have been made with IDL. It is similar in function to Matlab or Mathematica, though I do not know enough about those other programs to offer a detailed comparison. Two possibly helpful links are provided below.

• IDL Website: Official details and purchasing information. The entire program can be downloaded for free trial. These trials last seven minutes at a time (after it shuts off you can immediately restart it for another seven minutes). Some features are disabled without the license, such as the save feature. Buying a license does not change the software, it just unlocks all the features.
  
  
• Coyote's Guide to IDL Programming: The web site of a professional IDL consultant. This site has many examples of IDL coding and may be helpful in learning some good programming techniques.

NOTE: I am not getting anything from the IDL owners for this posting. I have written this because some people have asked about the references to IDL throughout the site. I am not compensated for using or writing about IDL. If this changes in the future, which would be cool but is nearly impossible, then I will announce such compensation.