Python - Random Forest - Iteratively adding trees

Good afternoon! Sorry for my English, but I need your help. I am doing some machine learning task on Python. I need to build RandomForest and than build a graph that will show how the quality of the training and test samples depends on the number of trees in the Random Forest. It is necessary each time to build a new Random Forest with a certain number of trees? Or i can somehow iteratively add trees (if it possible, can you give the exapmle of code how to do that)?

via Chebli Mohamed