Hi Bala,
CSS only allows you to influence appearance and some client side interactivity. Changing the dimension involves manipulating the underlying data source, which cannot be done via CSS. Since the standard Design Studio charts don't allow you to explicitly select one dimension from many, I don't think there's any alternative to your current approach of having a separate data source alias (connection) for each chart, even though the underlying query is the same for all.
I am not sure if Design Studio does any optimisation in cases where multiple data sources use the same underlying query but by the sound of your experience, it doesn't. Parallel query handling is planned for DS 1.5. This feature is likely to improve the performance of your dashboard.
Regards,
Mustafa.