Produce a empirical quantile-quantile plot plot
nan_qqplot(x) nan_qqplot(x,y) nan_qqplot(x,y,pvec)
specify the quantiles for plotting in a vector
nan_qqplot(x) display a quantile-quantile plot of x versus a normal distribution. If the distribution of x is normal, then the plot is linear.
nan_qqplot(x,y) display a quantile-quantile plot of x versus a quantile-quantile plot of y. If the distribution of x and y is the same, then the plot is almost linear.