add highlight for altair

app.py

@@ -78,6 +78,9 @@ def wb_ava_fig(vwater=1520., rwater = 1025., poissons=0.49):
     df = df[df["Angle"] < df["Ang_Crit"]]
     
     # Create the altair figure
+    highlight = alt.selection(type='single', on='mouseover',
+                          fields=["Vp sub-WB:Q"], nearest=True)
+
     base = alt.Chart(df).encode(
             x="Angle",
             y="Amplitude",