Add type information to expression columns

Expressions aren't columns, but they have an entry in the cols
property which currently doesn't have type info. This adds that which
allows it to be used as the Y-axis in graphs.

Fixes #3952

src/metabase/query_processor/annotate.clj

@@ -69,7 +69,10 @@
        :field-display-name (humanization/name->human-readable-name (:field-name this)))]
 
     metabase.query_processor.interface.ExpressionRef
-    [(assoc this :field-display-name (:expression-name this))]
+    [(assoc this
+       :field-display-name (:expression-name this)
+       :base-type          :type/Float
+       :special-type       :type/Number)]
 
     ;; for every value in a map in the query we'll descend into the map and find all the fields contained therein and mark the key as each field's source.
     ;; e.g. if we descend into the `:breakout` columns for a query each field returned will get a `:source` of `:breakout`