When, I fit an ARMA model to data, I look at the standardized residuals plot to assess if they behaves like uncorrelated random variables with zero mean and costant variance (if the model is good). Like this:
But this graph is useful also to assess the presence of GARCH effects? I don't understand: I want the residuals to be homoskedastic (so a random cloud, and no a systematic pattern in the plot), but this feature is not in constrast with the presence of a GARCH and so volatility clustering? In general, what information this plot gives to me, and what I expect if the model is good? Thanks!