It's just a bad error message. Mathcad 14 has lots of them. Never believe any error message from either a graph or a solve block. The problem is that you are passing a vector of values to the function df(x), which expects a scalar. Some Mathcad functions handle vectors of values automatically, but not all of them. If you evaluate df(x) outside the graph, that's the error you get. Use the vectorize operator on df(x) and it works. As an aside, you do not need to define x:X. In the definition of f(x) x is a proper argument, and just tells Mathcad where the argument in that location (in this case there is only one argument, but there could be many) must be used in the function. Once defined, you can call it with anything, including X. Richard