library(dygraphs)

lungDeaths <- cbind(ldeaths, mdeaths, fdeaths)

dygraph(lungDeaths, main = "Deaths from Lung Disease (UK)") %>%
  dyHighlight(highlightSeriesOpts = list(strokeWidth = 3)) -> d1

#this is a hack to set css directly
#  dyCSS designed to read a text css file
d1$x$css = "
.dygraph-legend > span {display:none;}
.dygraph-legend > span.highlight { display: inline; }
"

d1