Generated with the following R [r-project.org] code:

bitmap(file="PoissonCDF.png",type="png256",width=4,height=3,res=300,pointsize=12)
par(mar=c(3,3,1,1))

lambda <- c(4,1,10)
cols <- c("green","red","black")
plot(c(0,20),c(0,1),type="n")
for(x in 20:0) {
  for(i in 1:3) {
    lines(c(x,x+1),rep(ppois(x,lambda[i]),2),col=cols[i])
    points(x,ppois(x-1,lambda[i]),col="white",pch=19)
    points(x,ppois(x-1,lambda[i]),col=cols[i],pch=21)
    points(x,ppois(x,lambda[i]),col=cols[i],pch=19)
  }
}
legend(12,0.5,legend=c(expression (lambda==1),expression (lambda==4),expression (lambda==10)),col=c("red","green","black"),lwd=3,bty="n")
dev.off()