The type of plotly event. Currently 'plotly_hover',
'plotly_click', 'plotly_selected', and 'plotly_relayout' are supported.
source
a character string of length 1. Match the value of this string
with the source argument in plot_ly() to retrieve the
event data corresponding to a specific plot (shiny apps can have multiple plots).
session
a shiny session object (the default should almost always be used).