R中的三维散射点，黄土平滑曲线仅延伸到正值

我有以下代码：

library(readxl)
library(car)
library(rgl)


input <- read_xlsx("PlotsStatenIsland.xlsx")
Elevation <- c(input$Elevation)
ComNorm <- c(input$ComNorm)
Percent <- c(input$Percent)

Income2<-structure(list(X = NROW(Elevation), Elevation, Percent, ComNorm), .Names = c("X", "Elevation", "Percent", "ComNorm"), class = "data.frame")

scatter3d(ComNorm ~Elevation + Percent, data=Income2, fit="smooth",method="loess", df.smooth = 10, color="green",point.col="green", tickmarks="detailed",
          xlab="Elevation", ylab="Complaints", zlab="Percent Rise")

问题是曲线变成负值。有没有办法防止这种情况

这是一张照片：

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非常感谢

你好，马可。我添加了图像。但是，输入非常大