Thanks for your help! Here is a slightly modified code that produces the kind of figure I wanted:
p <- ggplot( data, aes(x, y) ) + geom_point_interactive( aes(data_id = facet) ) + facet_wrap_interactive( ~facet, interactive_on = "both", labeller = labeller_interactive( aes(data_id = facet) ) ) + theme( strip.background = element_blank(), strip.text.x = element_text( colour="black", size = 12) )
girafe( ggobj = p, options = list( opts_hover(""), opts_hover_inv(css = "opacity:0.1;") ) )
I think the problems were 1. mouse-over needs to be rather precisely on the text in the facet strip - here a single letter, 2. in the original code, data_id = facet was part of the global aestethics, but it seems it needs to be placed inside labeller_interactive to make the facets interactive
