同学你好，该知识点来自沪江网校《【Uni智能】英语零基础直达大学六级【9月班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
这个句子相当于：How good you have gotten at drawing is incredible.
How good you have gotten at drawing是主语从句，是整个句子的主语。
其中how good修饰have gotten，you是从句主语

类似的比如
It's still a puzzle how he became rich overnight.
=How he became rich overnight is still a puzzle
他是如何一夜暴富仍然是一个谜。

可以在这里看看主语从句的用法
http://www.hjenglish.com/new/p1161893/

祝同学学习进步~