Plants may be grafted in a multitude of ways and for many different reasons.
Since Jatropha does not grow in water logged area's, it would be interesting to find a rootstock that does not have that particular problem and graft Jatropha curcas on it.
So far Jatropha curcas has only been used as a rootstock for ornamental Jatropha integerrima because of its vigorous growth. (Photo AvP Indonesia 2007)
On the picture a so called wedge or cleft graft has been used. A=scion, B=rootstock.
This type of grafting is typical for herbaceous stems. Drawing from R.J.Garner, The Grafters Handbook.
If you think that grafting is not the proper method to produce millions of plants, do not forget that worldwide all citrus- and apple trees are grafted on rootstocks with known characteristics.