Fortunately this has been improved in the beta version of Graph 4.4, where the functions now work as you would expect. You can now use *g(a,b)=Sum(f(x),x,a,b)*. The step size is still not supported but I will consider adding it.

g(a,b)=Sum(f,a,b) <-as per help file

g(a,b)=Sum(f(),a,b)

g(a,b)=Sum(f(x),a,b)

produce an error msg. It appears the syntax must be

g(x,a,b)=Sum(f(x),a,b)

with x required as a dummy variable in the function name. But this means Sum is not useful in a graph equation, since x will be determined by Sum, not the x axis. Instead, a graph equation would have to call the custom function as g(0,x), with the first argument being an unused placeholder. Do I understand this correctly? It would be nice to specify the increment variable (and step value?) and not require the placeholder. Intuitively, I expected

g(a,b)=Sum(f(x),x,a,b[,step])

which could be used anywhere. An optional step value, though not essential, can be convenient.

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