plot: streamline error handling in examples

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