Roy Weintraub has made a very important point when reminding us about Sidney 
Weintraub's use of C+I and not "IS".
Indeed, the original diagram as presented by Hicks had a CC (MEC) schedule 
and not SI .
The original Harrod-Meade-Hicks equational and diagramatic system underwent 
many changes from its original nominal income- interest rate framework to 
what became the "trained intuition" of many, but not all economists, as Roy 
points out...
On these and related points, see Young (1987) and Darity and Young (1995)...

Warren Young