plot: streamline error handling in examples

[sbinet]

we should streamline and harmonize error handling in examples.
some are using:

if err != nil {
    panic(err)
}

while others are using log.Panic(err) and others are using log.Fatal(err).

I'd err on (consistently) using this kind of error handling:

if err != nil {
    log.Fatalf("could not do or perform foo: %+v", err)
}

(notice the %+v to be compatible with fmt.Errorf of Go-1.13 and golang.org/x/xerrors.Errorf for earlier Go versions)