added new error messages

pyicon/pyicon_plotting.py

 import sys, glob, os
+import warnings
 import json
 from pathlib import Path
 # --- calculations
@@ -1901,7 +1902,7 @@ def plot(data,
          grid_type='auto',
          # --- ckdtree interpolation
          res=0.3, fpath_ckdtree='auto',
-         coordinates='clat clon',
+         coordinates=None,
          antialias=1,
          mask_to_zero=False,
          # --- original grid
@@ -2128,12 +2129,54 @@ def plot(data,
           f"Unable to find file `fpath_ckdtree={fpath_ckdtree}`. \
           Try setting `fpath_ckdtree` explicitly."
         )
-
-    datai = interp_to_rectgrid_xr(
-        data.compute(), fpath_ckdtree,
-        lon_reg=lon_reg, lat_reg=lat_reg, coordinates=coordinates,
-        antialias=antialias, mask_to_zero=mask_to_zero,
-    )
+    
+    if coordinates is None:
+      # Infer the coordinates
+      if 'cell' in data.dims:
+        data = data.rename(cell='ncells')
+        coordinates = 'clat clon'
+      elif 'vertex' in data.dims:
+        data = data.rename(vertex='ncells') 
+        coordinates = 'vlat vlon'
+      
+      else:
+        warnings.warn(
+          "Coordinates are being inferred. If the resulting plot looks \n"
+          "unreasonable try setting `coordinates` kwarg explicitly."
+        )
+        if 'ncells' in data.dims:
+          coordinates = 'clat clon'
+        elif 'ncells_2' in data.dims:
+          data = data.rename(ncells_2='ncells') 
+          coordinates = 'vlat vlon'
+        else:
+          raise RuntimeError(
+            "Unable to infer coordinates. Please try setting the \n"
+            "`coordinates` key word argument explicitly."
+          )
+      
+    try:
+      datai = interp_to_rectgrid_xr(
+          data.compute(), fpath_ckdtree,
+          lon_reg=lon_reg, lat_reg=lat_reg, coordinates=coordinates,
+          antialias=antialias, mask_to_zero=mask_to_zero,
+      )
+    except IndexError:
+      raise ValueError(
+        "A problem was encountered attempting to interpolate the data to a \n"
+        "regular grid before plotting. This was *almost certainly* caused by \n"
+        "the incorrect specification of the `coordinates` keyword argument. \n"
+        "If the quantity you wish to plot is on cells it should be \n"
+        "`'clon clat'`, if on vertices `'vlon vlat'`."
+      )
+    
+    except AttributeError:
+      raise AttributeError(
+        "The ckdtree used for interpolating this object doesn't support \n"
+        f"interpolation from {coordinates} to 'lat lon'. You may need to \n"
+        f"recreate the ckdtree stored in {fpath_ckdtree} or specify a \n"
+        "different ckdtree."
+      )
 
     lon = datai.lon
     lat = datai.lat