SSRS - Graph Line Value for Parameter
You need to refer to the parameters in your main sql query. 

Something like: 

Where databaseName = @databases 

Or if you want to be able to select multiple databases.

Where databaseName in (@databases)

Here is an example using date parameters and multi-select parameters in a report https://technet.microsoft.com/en-us/library/aa337396(v=sql.105).aspx

If you Google " ssrs parameter sql query" there are lots of working examples and even some youtube videos giving step by step instructions.