org-plot.el: Make min/max keywords consistent

* lisp/org-plot.el: (org-plot/add-options-to-plist): Have both x/y
min/max, but have min/max aliased to the y-axis options.

lisp/org-plot.el

@@ -65,8 +65,10 @@ Returns the resulting property list."
 		("timeind"   . :timeind)
 		("timefmt"   . :timefmt)
 		("min"       . :ymin)
-		("max"       . :ymax)
 		("ymin"      . :ymin)
+		("max"       . :ymax)
+		("ymax"      . :ymax)
+		("xmin"      . :xmin)
 		("xmax"      . :xmax)
 		("ticks"     . :ticks)
 		("trans"     . :transpose)