The Falang language manager intercepts all database queries made on the front end of a site in order to retrieve different versions of content for different languages. It works well for most "normal" database queries, but it does not work properly for some queries that may be used for charts. For example, with Falang installed, many of the sample charts in Plotalot will work perfectly in the back end of Joomla, but in the front end they return errors such as "This chart requires 2 columns".
The solution is to create a Plotalot Database object for the current site, and set your charts to use this database instead of the default of "This site". Plotalot will then make a separate connection to the database, bypassing Falang. You can give this Plotalot Database any name you like, for example "Direct connect". You need to specify the host, database name, user name and password. Use the test button to make sure you got everything correct. Then select this new Database for any charts that are not working properly in the front end.