I think SQL can be used, so I just use SQLite for the data visualization. I wrote a extension to do so, but many thing it does not yet include. I think SQLite extensions for doing graphics will be good idea.
Can you elaborate on how exactly you're "using SQLite for data visualization?"
The data is either stored in a SQLite database or can be imported (either permanently or temporarily), and then queried (including with a WHERE clause and/or various other stuff, if applicable), and extension for graphics can be use for making the graphics (sqlext_graph), display on screen (sqlext_xwindows), or save to disk (sqlext_pipe and Farbfeld Utilities). Mainly, I fail to see why should I need a different program, if SQL seems good enough? (Of course, if other programs are more helpful to you, you can use those other programs, either instead of or in addition to, SQL. For example, I think sometimes SQLite is combined with R for statistics and graphics.)