Shift table rows up and down

Hi
Based on dropdown item selection,some rows have to visible and some other rows invisble.
Iam able to make thehtml table  rows visible and invisible using javascript and calling tht js function on the dropdown selection index changed.
But when  4th and 5th rows are invisble then the gap between 2 nd and 6th rows remains as it is.How can i shift the 6th and 7th rows up and down based on the  drop down selection
Thanks and Regards
 
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