fix the overlap issue without group

[xiangpin]

The related issue

> library(ggplot2)
> library(scatterpie)
> df<-data.frame(lat=c(1,1.5),long=c(2,2.1),A=c(0.2,0.5),B=c(0.7,0.4),C=c(0.1,0.1))
> ggplot()+geom_scatterpie(aes(x=long,y=lat),cols=c("A","B","C"),data=df,pie_scale=150)+coord_equal()