Coming soon: chart selections

[jrieke]

Hey all!

We’re excited to share a prototype for chart selections. This new feature will let your users select data points or areas in charts and return expose that event on the Python side. The current prototype works for both Plotly and Altair charts.

You can find the demo app with instructions here: https://chart-selections-demo.streamlit.app/

How it works

TLDR: It works just like any other input widget!

To enable selections for your chart, you need to use the new on_select parameter for
st.plotly_chart or st.altair_chart. There are 3 ways to use this parameter and
retrieve the selection data:

1. Using return values: Set on_select="rerun" or on_select=True. Then,
   Streamlit will rerun the script whenever you select a point in the chart, and
   return the selected data in a dict-like object:

   event_data = st.plotly_chart(fig, on_select="rerun")
   st.write(event_data)
   

   This works just like a return value you’d get from a slider or text input! The
   selection data is stored under event_data["select"]. You can also use dot
   notation for this dict (e.g. event_data.select), similar to how
   st.session_state or st.secrets work.

2. Using session state: If you additionally set the new key parameter, Streamlit
   will store the event data in session state (in addition to returning it):

   st.plotly_chart(fig, on_select="rerun", key="my_chart")
   st.write(st.session_state.my_chart)
   

   This is the same behavior as for any other input widget if you set key. The
   session state value is just the same dict as the return value (event_dict
   in the example above).

3. Using callbacks: You can also pass a callback function to on_select. Just like
   with other callbacks in Streamlit (e.g. on_change), this callback function
   will be executed before the rest of the script reruns:

   def callback():
       st.write("Hello from the callback!")
       st.write(st.session_state.my_chart)
   
   st.plotly_chart(fig, on_select=callback, key="my_chart")
   

Timeline

We’re finishing up the implementation now and are thinking through how we can extend this API to other selections/events in the future. This will take a bit, so I’d guess we will release an experimental version in the next 1-2 months. There are also a few improvements (e.g. making chart selections work with forms or with app testing) that we might ship as follow-ups.

Feedback

Please let us know what you think, no matter if positive or negative. You can comment below or on the GitHub issue. Also, let us know if you find any bugs!

Happy Streamlit-ing!