Comparison of minimiser convergence speed and robustness. The dotted lines show the χ² progress towards its minimum as a function of the number of calls, until the algorithm stops (cross symbol). Several algorithms are shown, namely Powell (blue), GC (red), and MINUIT (green), and the horizontal dash-dotted lines highlight the best-fit obtained for each algorithm. Ten different starting points are run, and the distribution of the best found are projected on the right-hand panel to show that sometimes the algorithm is stuck in a local minimum. From top to bottom: number of nuisance parameters in the model increases. See text for discussion.