add a method to Figure that takes in new data

[mikedewar]

The new method should look like

data = DataFrame({'y' = [1,2,3], 'x' = [1,2,3]})
p = Figure(data)
p += Line(x='x', y='y')
p += DataFrame({'z' = [3,4,5], 'x' = [2,3,4]})
p += Line(x='x', y='z')

and the internal data frame should have merged the xs and added missing data for the ys and zs that don't have data points.

[mikedewar]

Panda's combine does the trick nicely:

combiner = lambda x, y: np.where(isnull(x), y, x)
df1.combine(df2, combiner)

[mynameisfiber]

That's great! One thing though: do we want all this functionality by massively overloading the addition operator?

One consequence of this would be many isinstance checks in __add__ which I find incredibly ugly! Maybe we can have reasonable helper functions that do this data combination for you. The main thing I don't like about overloading add this way is that you never know what the result of the operation is: it does drastically different things based on the incoming type. (this is one of the things I hate most about my implementation of the javascript module).

So, any thoughts?