Give anotation for plot

this plot should have notes that explain the value. I can't understand what the value mean on this plot

OK, will add description. To immediately answer the question, the value is the mean applied to a monotonic interpolation function. The code from that commit was:

a = alt.Chart(resp_nwm).mark_square(size=40).encode(
    x=alt.X('variable:N', scale=alt.Scale(rangeStep=80), axis=alt.Axis(labelAngle=0)),
    y=alt.Y('value')
    ...
b = a.mark_line(opacity=0.8, interpolate='monotone').encode(
    x=alt.X('variable')
)

But it's a good idea, and I think it's worth the effort to add a description