qplot的错误信息求解释

tongjiyyg

Error in smooth.construct.tp.smooth.spec(object, dk$data, dk$knots) :
  A term has fewer unique covariate combinations than specified maximum degrees of freedom
错误于if (nrow(layer_data) == 0) return() : 参数长度为零
是什么意思
哪位牛人可以解释否

zcvb6

完整code发来看看