address @kelle's comments

Author: keflavich

tutorials/position-velocity-diagrams/PVDiagramPlotting.ipynb

@@ -19,7 +19,9 @@
     "cube, pv-diagram\n",
     "\n",
     "## Summary\n",
-    "In this tutorial, we will extract position-velocity (PV) diagrams from a cube and plot them."
+    "In this tutorial, we will extract position-velocity (PV) diagrams from a cube and plot them.\n",
+    "\n",
+    "Position-velocity diagrams are often used in radio astronomy for analysis of rotating objects, like protostellar disks and galaxies, to measure rotation curves and determine the contained mass.  They are also used in studies of atomic and molecular clouds to show where overlapping emission may point at interactions between distinct clouds.  Both radio and optical position-velocity diagrams are used to study outflows, jets, and winds; in the optical, two-dimensional spectra obtained from long-slit spectrographs naturally produce the equivalent of a position-velocity diagram."
    ]
   },
   {
@@ -98,7 +100,9 @@
    "source": [
     "# PV Extraction from Pixel Coordinates\n",
     "\n",
-    "First we create an extraction path.  The entries are pairs of pixel coordinates, (x,y)"
+    "First we create an extraction path.  This path is the spatial coordinates through which we are cutting out our PV diagram.\n",
+    "\n",
+    "The entries are pairs of pixel coordinates, (x,y).  In this case, we've selected arbitrary coordinates for our demonstration, but for your use case, you might pick the axis of an outflow, the major axis of an inclined disk, or a spiral following an arm of a galaxy."
    ]
   },
   {
@@ -527,13 +531,32 @@
    "source": [
     "pvdiagram.writeto(\"saved_pvdiagram.fits\", overwrite=True)"
    ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "ffb2c3bb-c95d-4a24-bd5b-6c08d8fa56ba",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "cube2 = SpectralCube.read('https://dataverse.harvard.edu/api/access/datafile/:persistentId?persistentId=doi:10.7910/DVN/YUNZ1F/OJJTGR', format='fits')\n",
+    "cube2"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "3e217d0d-fdfb-4628-9fbc-29696ce5d3c7",
+   "metadata": {},
+   "outputs": [],
+   "source": []
   }
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Py 3.9",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "python39"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
@@ -545,7 +568,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.2"
+   "version": "3.9.7"
   }
  },
  "nbformat": 4,