Fix units / res args for *png* plotting device.

Update the `units` argument so that it accepts values from ["px", "in",
"cm", "mm"].

The `res` argument was added for the cases where `units` is specified
and is not default (px). A default value of 72 is provided by rmagic in
the case that `units` is not "px" but `res` is not specified.

IPython/extensions/rmagic.py

@@ -323,9 +323,13 @@ def Rget(self, line):
         help='Convert these objects to data.frames and return as structured arrays.'
         )
     @argument(
-        '-u', '--units', type=int,
+        '-u', '--units', type=unicode, choices=["px", "in", "cm", "mm"],
         help='Units of png plotting device sent as an argument to *png* in R. One of ["px", "in", "cm", "mm"].'
         )
+    @argument(
+        '-r', '--res', type=int,
+        help='Resolution of png plotting device sent as an argument to *png* in R. Defaults to 72 if *units* is one of ["in", "cm", "mm"].'
+        )
     @argument(
         '-p', '--pointsize', type=int,
         help='Pointsize of png plotting device sent as an argument to *png* in R.'
@@ -507,7 +511,12 @@ def R(self, line, cell=None, local_ns=None):
                     val = self.shell.user_ns[input]
                 self.r.assign(input, self.pyconverter(val))
 
-        png_argdict = dict([(n, getattr(args, n)) for n in ['units', 'height', 'width', 'bg', 'pointsize']])
+        if getattr(args, 'units') is not None:
+            if args.units != "px" and getattr(args, 'res') is None:
+                args.res = 72
+            args.units = '"%s"' % args.units
+
+        png_argdict = dict([(n, getattr(args, n)) for n in ['units', 'res', 'height', 'width', 'bg', 'pointsize']])
         png_args = ','.join(['%s=%s' % (o,v) for o, v in png_argdict.items() if v is not None])
         # execute the R code in a temporary directory