use cf-xarray's registry in the example (#128)

* use the unit registry provided by cf-xarray instead of modifying the attrs

* install cf-xarray into the docs environment

* update whats-new.rst

* import cf-xarray before pint-xarray

Author: keewis

docs/examples/plotting.ipynb

@@ -16,6 +16,8 @@
    "outputs": [],
    "source": [
     "import xarray as xr\n",
+    "# to be able to read unit attributes following the CF conventions\n",
+    "import cf_xarray.units  # must be imported before pint_xarray\n",
     "import pint_xarray\n",
     "from pint_xarray import unit_registry as ureg\n",
     "\n",
@@ -42,29 +44,6 @@
     "data"
    ]
   },
-  {
-   "cell_type": "markdown",
-   "id": "medium-backup",
-   "metadata": {},
-   "source": [
-    "## convert units into a format understood by pint\n",
-    "\n",
-    "<div class=\"alert alert-info\">\n",
-    "<strong>Note:</strong> this example uses the data provided by the <code>xarray.tutorial</code> functions. As such, the <code>units</code> attributes follow the CF conventions, which <code>pint</code> does not understand by default. To work around that, we are modifying the <code>units</code> attributes here, but in general it is better to use a library that adds support for the units used by the CF conventions to <code>pint</code>.\n",
-    "</div>"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "id": "published-powell",
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "data.lat.attrs[\"units\"] = \"degree\"\n",
-    "data.lon.attrs[\"units\"] = \"degree\""
-   ]
-  },
   {
    "cell_type": "markdown",
    "id": "banned-tolerance",

docs/requirements.txt

@@ -2,6 +2,7 @@ pint>=0.14
 xarray>=0.16.0
 pooch
 netCDF4
+cf-xarray>=0.6
 sphinx<4
 sphinx_rtd_theme
 ipython

docs/whats-new.rst

@@ -11,6 +11,8 @@ What's new
 - convert the note about dimension coordinates saving their units in the attributes a
   warning (:issue:`124`, :pull:`126`)
   By `Justus Magin <https://github.com/keewis>`_.
+- use ``cf-xarray``'s unit registry in the plotting example (:issue:`107`, :pull:`128`).
+  By `Justus Magin <https://github.com/keewis>`_.
 
 0.2 (May 10 2021)
 -----------------