Add title to the plotly legend

This functionality has since been included within the layout function in the legend option. There's a sub-option called title within which you can supply a list that includes the text.

mtcars %>% 
  plot_ly(x = ~disp, y = ~mpg, color = ~factor(cyl), size = ~wt) %>% 
  add_markers(hoverinfo = "text",
              text = ~paste("Displacement = ", disp, "\nMiles Per Gallon = ", mpg)   ) %>% 
  layout(title = "Custom Hover Text", 
         legend = list(title = list(text = "<b>Cylinders</b>"))) # TITLE HERE

The only way I know is to use an annotation and add it to the plot. Like this:

legendtitle <- list(yref='paper',xref="paper",y=1.05,x=1.1, text="Cylinders",showarrow=F)

mtcars %>%  plot_ly(x = ~disp, y = ~mpg, color = ~factor(cyl), size = ~wt) %>%   
  add_markers(  hoverinfo = "text",
                text = ~paste("Displacement=",disp, "\nMiles Per Gallon = ", mpg)) %>%   
  layout(title ="Custom Hover Text", annotations=legendtitle )

Yielding:

It is a bit tricky to place the legend title though, not sure if this placement would always work.

Another way would be to use ggplot and ggplotly of course, and let ggplot figure it out.