I often find myself with a lot of multivariate time series data. It's also usually quite noisy, which makes for hard-to-interpret plots. Taking a simple moving average over the variables is a good way to smooth things out.
I use the Awk script below to process my data files, which normally have a format in which the first column is time and the remaining columns contains the value of each variable. The script will output the first column untouched, and will take an average over a sliding window to compute a new value for each variable. It isn't particularly sophisticated, but it usually gets the job done for ad hoc data visualizations.