The well to wheels analysis is complicated, but I have confidence in the NRDC-EPRI study which you can easily get at
Miastrada Motors - References (I think the full report is reference (3) )
Figure 5-1 shows that converting a gasoline hybrid to plug-in results in more CO2 in a coal powered scenario than the production hybrid.
This result has confounded NRDC and the plug-in guys ever since. The conclusions of the report try mightily to spin things differently. And they assume the world is going to get off coal almost immediately, so the words can be encouraging to plug-in guys.