CsegGraph: a graph colouring instance generator. The implementation of a graph generator associated with the estimation of mathematical derivatives is described. The graph colouring instances are obtained as intersection graphs G Π (A) of the sparsity pattern of A∈ℜ m×n with row partition Π. The size of the graph is dependent on the row partition; the number of vertices can be varied between the number of columns (using single block row partition Π=Π 1 ) and the number of nonzero entries of A (using m-block row partition Π=Π m ). The chromatic number of the generated graph instances satisfy χ(G(A))≡χ(G Π m (A)). A summary of colouring results on the ColSegSet problems is presented. We also show that the main idea of the graph instance generator can be extended to general undirected graphs