Since these walls are 12" of fibreglass, I'd expect that graph to be a pretty straight line.
No.
Every component of a wall assembly has some R-value. To map the heat loss, you have to account for everything in the wall. And in this case there isn't a single 12-inch batt, there's an existing wall with unspecified thickness batts, then a new, inner wall with 8-inch batts. There's presumeably an air space between them, and that air space (believe it or not) has an R-value. So does the interior finish (sheetrock?), and so does whatever forms the outer face of the existing wall. The MOIST calculator from NIST even included an R-value for the still air layer at the outer face of the exterior siding.
Here's an example of a table the provides some of the values used. Note that they list an R-value for the air film on the interior face as well as for the exterior face. Curiously, this table doesn't include gypsum wallboard. You could use plaster sheets, I suppose, but I would look for a different table that actually lists sheetrock. This is just an example:
https://efficiencymatrix.com/building-material-r-values/
Take a common house exterior wall (old style), for example:
Indoor air film = 0.14
1/2" GWB = 0.075
5-1/2" batt insulation = 19
1/2" plywood sheathing = 0.10
Vinyl siding - 0.009
Outdoor air film = 0.04
So the total R-value of the wall is the sum of all these materials ==> 19.364
Assume an indoor temperature of 70 degrees F and an exterior temperature of 0 degrees F. That makes delta T 70 degrees. Divide that by 19.364 and we get a temperature change of 3.615 degrees per R-1. The temperatures at each interface are computed proportionally. When its drawn graphically, you can put the temperature on the vertical (Y) axis. Take the dew point, extend a horizontal line across from that temperature on the Y axis, and it shows you where in the wall the moisture will start to condense.