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Through this dialog it is possible to choose which optimizer engine is used in the fit, control the parameters value and also fixing them by unchecking the appropriate checkbox under the Fit column next to the parameter's name.
If the Fit checkbox in the lower part of the dialog is unchecked (like in the picture above) it meas that the fit has not been performed (or that the parameters have been changed). To perform the fit just click on it. The status bar should show the progress of the fit. When finished the chi squared of the fit should be provided.