Legend title in plotly

Question

How do I specify title for legend in plotly? I have a stacked bar graph that plots different durations like 0-10, 11-20. I want the legend title to say 'Duration'.

Solution

The simplest way to specify a legend title is to set it via ggplot and have plotly read it from the corresponding object:

library( plotly )

gg <- ggplot( mtcars, aes( x=mpg, y=wt, color=factor(vs) ) ) +
  geom_point() + labs( color = "MyTitle" )
ggplotly( gg )

However, the problem is that plotly converts the legend title into an annotation, which becomes disconnected from the legend in the process. In my browser, it also overlaps with the plotly menus in the top right corner:

To get around this problem, you can remove the legend title from the ggplot object altogether and add the annotation by hand yourself:

gg <- ggplot( mtcars, aes( x=mpg, y=wt, color=factor(vs) ) ) +
  geom_point() + theme( legend.title = element_blank() )
ggplotly( gg ) %>%
  add_annotations( text="MyTitle", xref="paper", yref="paper",
                  x=1.02, xanchor="left",
                  y=0.8, yanchor="bottom",    # Same y as legend below
                  legendtitle=TRUE, showarrow=FALSE ) %>%
  layout( legend=list(y=0.8, yanchor="top" ) )

Note that the same y coordinate is used for both the title and the legend, but the former is anchored at the bottom, while the latter is anchored at the top. This keeps the title from being "disconnected" from the legend. Here's what the final result looks like: