Last Update: 2/4/2015
Open the NLFit dialog, by selecting Analysis: Fitting: Nonlinear Curve Fit.
When starting from a graph, on the Data Selection / Settings tab, the Input Data node will have a Range defined for each curve in the active layer of the graph. (If you start from the worksheet, first highlight the multiple columns (the replicate data), and then open the NLFit dialog.)
Use the Multi-Data Fit Mode drop-down control to specify how the multiple datasets are treated. Choose Concatenate Fit, to concatenate all Y columns and fit as one curve.