Methods for function levelplot and zoo and solaR objects.
signature(x = "formula", data = "zoo"): The zoo object is converted into a data.frame object and
additional columns are added (day, month and
year, and w with the solar hour in radians).
This data.frame is the data argument for a
call to levelplot, using the S3 method for class formula.
signature(x = "formula", data = "Meteo"): The Meteo object is converted into a zoo object, and
the previous method is used.
signature(x = "formula", data = "Sol"): idem
signature(x = "formula", data = "G0"): idem