Following is what Chris says on his blog about this graph : 

“Here’s some results from the Torqeedo speed tests done just before hauling her out.  These results are wind and tide effected but include readings taken in both directions and I’m hoping they would balance.  The curve fitting is done by the charting software and I’ve chosen the exponential regression option because I think it should be an exponential shape.  That is, as speed increases the power required increases by an exponential amount.  It looks about right to me.”