col_wrap arg to da.plot() can be a string?

What happened:
if you try to plot a data array with 3 dimensions but the third dim has length 1, then col_wrap accepts a dim name and plots facets with pcolormesh. according to the documentation col_wrap argument should be an int so I assume this is unintentional http://xarray.pydata.org/en/stable/generated/xarray.DataArray.plot.html

if dim 3 has length >1, then col_wrap still takes a str but uses hist()

What you expected to happen:
I expected an error if col_wrap is not an int. I also expected da.plot(col='3rd_dim') to give me one facet if the dim has length 1 - this (mis)behavior is described in #620

all a bit confusing :). I thought I understood plotting but then ran into bug #620 and started fumbling around. I am trying to plot a dataarray in a function but I don't know ahead of time how many values 3rd dim has. I also wanted to avoid limiting my function to pcolormesh() only. da.plot.pcolormesh(col='3rd_dim') works to plot the images regardless of dim length.

Minimal Complete Verifiable Example:

da_list = []
for i in np.arange(1,3):
    data = np.ones((64, 48), dtype = 'uint8') * (200-i*15)
    oneframe = xr.DataArray(
        data=data[..., None],
        dims=('x', 'y','measurement'),
        coords={'measurement':[f't{i*40}']}
    )

    da_list.append(oneframe)

twoframes = xr.concat(da_list, 'measurement')

oneframe.plot(col_wrap='measurement') # Oddly, facets with pcolormesh()
twoframes.plot(col_wrap='measurement') # Oddly, a single facet with hist()
oneframe.plot(col='measurement') # ValueError: IndexVariable objects must be 1-dimensional ala issue #620
twoframes.plot(col='measurement') # facets with pcolormesh()
oneframe.plot.pcolormesh(col='measurement') # facets with pcolormesh()
twoframes.plot.pcolormesh(col='measurement') #facets with pcolormesh()

Anything else we need to know?:
@nfahlgren noted a workaround using FacetGrid directly

from xarray.plot.facetgrid import FacetGrid
gd = FacetGrid(data=oneframe, col="measurement")
gd.map_dataarray(xr.plot.imshow, "x", "y")

is consistent with:

gd = FacetGrid(data=twoframes, col="measurement")
gd.map_dataarray(xr.plot.imshow, "x", "y")

Environment:
Windows 10, python 3.7

Start by running the minimal reproducible example against DataArray.plot, including pcolormesh, hist, and xarray.plot.facetgrid.FacetGrid. Trace how col and col_wrap are dispatched for one- and multi-value dimensions. Done means non-integer col_wrap is rejected and the requested single-value col behavior is consistent with the documented plotting API.