The residual plot reflects how the whole 3D field converges along with the iterations. Normally the line drops dramatically after initial a few iterations and eventually it is likely to level out. The values shown in the residuals plot are in the result file in the windfield folder of your project. It is normal ascii.
Look for the line:
residuals vs sweep or iteration number
ISWP P1 U1 V1 W1 KE
1 1.2699E+06 1.8007E+09 5.0084E+16 1.6278E+10 5.0329E+18
2 1.1060E+06 3.0985E+15 3.6667E+05 7.4362E+14 3.7743E+06
3 6.0253E+05 4.2483E+08 1.8571E+06 2.6402E+08 1.0966E+07
4 3.4577E+05 1.5243E+08 2.4090E+06 1.4729E+08 1.1876E+07
5 2.3887E+05 1.2430E+08 2.5225E+06 1.2388E+08 1.1024E+07
ISWP P1 U1 V1 W1 KE
1 1.2699E+06 1.8007E+09 5.0084E+16 1.6278E+10 5.0329E+18
2 1.1060E+06 3.0985E+15 3.6667E+05 7.4362E+14 3.7743E+06
3 6.0253E+05 4.2483E+08 1.8571E+06 2.6402E+08 1.0966E+07
4 3.4577E+05 1.5243E+08 2.4090E+06 1.4729E+08 1.1876E+07
5 2.3887E+05 1.2430E+08 2.5225E+06 1.2388E+08 1.1024E+07
The scaling is that it finds out the maximum value in these lines and the minumum value. Then all the other values are scaled according to that.
The residual is calculated over the whole area. Please have a look at the description here.