x <- seq(-5,10,0.4)
y <- 1:length(x)
for (i in 1:length(x)){
  y[i] <- (sum(x[1:i]))^2
}
par(mfrow=c(1,1),mar=c(2,6,1,4),cex=1.2)
plot(x,y,ylab=c(expression(y == (sum(x[j], j==1, i))^2)))