The best Statistical Analyzer alternative to SigmaPlot is LabPlot, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to SigmaPlot and four of them are Statistical Analyzers so hopefully you can find a suitable replacement. Other interesting Statistical Analyzer alternatives to SigmaPlot are NumeRe, GraphPad Prism and Derive.
LabPlot is a free, open source and cross-platform Data Visualization and Analysis software accessible to everyone and trusted by professionals.
Open source framework for numerical computations and advanced statistical analysis, featuring nonlinear curve fitting, ODE solving, 1D/2D publication-quality visualization, Fourier and wavelet transforms, intuitive syntax, and namespace-based code organization for research.
GraphPad Prism is a powerful combination of basic biostatistics, curve fitting and scientific graphing in one comprehensive program. More than 100,000 scientists in over 100 countries rely on Prism to analyze, graph and present their scientific data.
Derive 6.1: Powerful system for doing symbolic and numeric mathematics. This application is no longer available or supported.
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There is no option to write your own equations and fit a curve to that in Prism, although it's great for general graph plotting.