Don't squeeze DataArray before plotting

[jhamman]

As was discussed in #608, we should honor the shape of the DataArray when selecting plot methods. Currently, we're squeezing the DataArray before plotting. This ends up plotting a line plot for a DataArray with shape (N, 1). We should find a way to plot a pcolormesh or imshow plot in this case. The trick will be figuring out what to do in _infer_interval_breaks.

[shoyer]

I see two options:

1. Squeeze after the row/col selection logic.
2. Raise an error if someone attempts to plot a (N, 1) plot as 2D.

1 might actually be preferable to not squeezing at all.

[stale]

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[dschneiderch]

I already mentioned this issue in a couple others but, as an FYI, I ran into this trying to write a plotting function that accepts a dataarray and **kwargs. It's a bit annoying to have to deal with the dataarray differently depending on whether the facetting variable has length == 1 or >1.

[shoyer]

I would support a breaking change to Xarray's plotting methods here to remove squeezing. These sort of inconsistencies are really indeed annoying to program around!

[headtr1ck]

Reviving this ancient issue here.
We could add a deprecation cycle and issue a warning for dimensions with size 1.