Here's another way to plot the result.
I've modifed the R value calculation to help convergence.
Notice I've change the analysis to .tran and added a .meas statement.
There are some other mods but I'll let you review the schematic I posted.
Anyway, Run the simulation (it takes about 5 minutes to complete).
After it completes:
1. View->Spice error log.
2. Rht-clk the any of the ".step r=nnn" row entries
3. select "Plot .step'ed .meas data" from the pop-up menu.
a new graph will appear
4. Rht-clk in the new graph
5. Select "Visible Traces"
6. Select "Out"from the pop-up menu, to add the Out measurement to the graph
The Output should now be shown in the graph.
After doing step 6, the new graph should show a decreasing R value in the horizontal axis and the Output voltage in the vertical axis.
See attached.