Well having multiple JSON parsers in Ruby on Rails is not a lot of fun.

Let me explain the situation. I was happily using “.to_json” (means Rails JSON parser) until I had to parse a POST request with JSON body.
I googled it like normal people do and found this:

require "json"
data = JSON.parse(input)

which worked fine until I tried my Open Flash Charts. They were blank. No error message. Nothing.
Removing the statement to require JSON fixed the issue.

I also tried

require "json/pure"

and

require "json/ext"

with similar results.

The short debugging session revealed that Ruby Open Flash Chart 2 helper is generating right JSON when Rails built-in generator is used.
When using json, json/pure or json/ext the certain characters (slashes, quotes) in a resulting string were escaped (sometimes several times) which breaks the Open Flash Charts.

The solution was to use

data = ActiveSupport::JSON.decode(input)

which is a call to parse the json string using the Rails built-in parser.