[API] Should encode() overwrite or update?

Consider this:

layer = Layer(data)
layer.encode(x='Horsepower')
layer.encode(y='Miles_per_Gallon')

Currently, this result is identical to

Layer(data).encode(y='Miles_per_Gallon')

this is because every time encode is called, it overwrites the current encoding.

I think it might be less confusing to change encode so that it instead updates the encoding, such that the above would be equivalent to

layer = Layer(data).encode(
   x='Horsepower',
   y='Miles_per_Gallon'
)

Thoughts?

[ellisonbg]

I think if someone is iterating on a single Layer during exploration, it is very likely they will want to re-encode a single channel. Because of this, we shouldn’t warn upon re-encoding a channel.

[jakevdp]

closed by #103