What is the historical spread between corporate bonds and government bonds?

Besides the government curve and the LIBOR curve there are corporate (company) curves. These are constructed from the yields of bonds issued by corporates. As corporates have lower creditworthiness than governments and most large banks these yields are typically higher. Corporate yield curves are often quoted in terms of a "credit spread" over the relevant swap curve. For instance the five-year yield curve point for Vodafone might be quoted as LIBOR +0.25%, where 0.25% (often written as 25bps or 25 basis points) is the credit spread.