I am trying to use the 'Regression Wizard' in SigmaPlot to obtain a best-fit curve to a data set of ~15 data points that map out a spiral. None of the equation categories given in the regression wizard seem to work (i.e. polynomial, exponential, logarithm, hyperbola etc. etc.) and I am unsure how to set a user defined equation to the data set. It's a logarithmic spiral so I'm assuming the eqn in polar co-ords should be; r(theta)=a*exp(b*theta) parametrically it should be; x=r*cos(theta)=a*cos(theta)*exp(b*theta) y=r*sin(theta)=a*sin(theta)*exp(b*theta) and in cartesian co-ords; x^2+y^2=exp((theta)*cot(a)) Any advice on SigmaPlot or plotting spirals in general will be greatly appreciated!!!