Apparantly, the Leaf doesn't emit more than a Mustang.
Car emissions calculator UK - compare total CO2 and NOx emissions
Plug in emissions comes down to their efficiency and what they are charging off of. Lots of coal burned for their power, and yes they can be dirty. Renewable and nuclear thrown into the mix and they'll clean up really quick.
There two big advantages to plug ins over the ICE. One is that the owner has an option in most regions in buying cleaner electricity, or making it themselves. While biofuels could possibly be made or bought by a person for the ICE, virtually all cars on the road can't use 100% biofuels. Some can't go higher than 5 or 10 percent. Then the fuels may only reduce the amount of NOx and particulates, if at all.
The other advantage is that the EV emissions are at the power plant, and not in the middle of town as with an ICE.
The EPA has more limited info in the side by side comparisons under the environment tab. The site doesn't include the car's manufacturer emission, and only goes beyond the tailpipe for CO2.
CO2 emissions are directly related to fuel economy. The amount of carbon per unit of fuel type is a known. So it just some basic math getting an users CO2 emissions from their fuel efficiency.
The other emissions reported are estimated from the amounts measured from the official tests, which are measured in g/mile or kilometer. Digging the actual values of a model from the EPA will be a pain, but a high estimate can be reported based on the car's emission bin. The only hiccup there is that manufacturers tend to certify their cars for a worse emission bin in federal states vs. CARB ones, even if the car has all the same equipment. It saves them some on warranty costs.