Hi @shoyer , here's an attempt: https://github.com/pydata/xarray/pull/794

Here is a mwe:

``` python import xarray as xr import numpy as np

temp = xr.DataArray(np.random.randn(3, 5), dims=['time', 'x'], coords={'x': [1,2,3,4,5], 'time': [1,2,3]}) temp.coords['rtime'] = temp.time + 3

for some reason this works

temp.plot(x='x', y='rtime');

this does not

temp.plot(x='rtime', y='x');

and this works again

temp.T.plot(x='rtime', y='x'); ```

the question (I guess) is: is it legal in xarray to use a non-linked coordinate name as argument for a plot? For example @jamesp wants to plot the following DataArray:

python <xarray.DataArray 'temperature' (time: 3, x: 5)> ... Coordinates: * x (x) int64 1 2 3 4 5 * time (time) int64 1 2 3 r int64 2 rtime (time) float64 0.5 1.0 1.5

By doing:

python .plot(x='x', y='rtime');

instead of the more conventional:

python .plot(x='x', y='time');

The dimension is still time but the coordinates of the plot (for the axis) is rtime. It works if you don't change the order of the dims, but it does not work if you do (hence the transpose in my workaround).

I think that the first case works by chance, since the data dimensions are still 'time', 'x'. But this workaround should work:

python dat.temperature.sel(r=2).mean('y').T.plot(x='rtime', y='x')

(note the transpose)

I guess that what you are asking is that xarray should infer from y='x' that the other dimension must be time, and then scale with your new rtime coordinates?

Aha, I think I just got it. I deleted my previous answer, as I don't know how to answer your question :flushed:

Just out of curiosity, what is the use case here? In your example the coordinate rtime has a shape (time: 3, r: 3) ant it is not linked to any dimension of the variable that you are plotting. What do you expect the plot to do?