QtiPlot includes quick access to the most useful functions for fitting.

This command is used to fit a curve which has a linear shape.

The results will be given in the Log panel:

This command is used to fit a polynomial function to data which has a curvilinear shape. It opens the Polynomial Fit Options dialog, allowing you to choose the curve to fit, the order of the polynomial function to use, the number of points of the resulting curve and the abscissa limits for the fit.

The results of the fit are displayed in the Log panel

**Figure 6-5. The results of a Fit Polynomial..., showing the initial data, the curve added to the plot, and the results in the log panel.**

This command is used to fit a curve which has a sigmoidal shape. The function used is:

in which A_{1} is the low Y limit, A_{2} is the high Y limit, x_{0} is the inflexion (half amplitude) point and dx is the width.

When the X axis is using a logarithmic scale, the **Fit Boltzmann (sigmoidal)** command uses the
Logistical equation for fitting:

where A_{1} is the initial Y value, A_{2} is the final Y value, x_{0} is the inflexion point (center) and p is the power.

This command is used to fit a curve which has a bell shape. The function used is:

in which A is the height, w is the width, x_{c} is the center and y_{0} is the Y-values offset.

This command is used to fit a curve which has a bell shape. The function used is:

in which A is the area, w is the width, x_{c} is the center and y_{0} is the Y-values offset.

This command is used to fit a curve with a Pseudo-Voigt function which is a linear combination of Gaussian and Lorentzian functions:

The parameters of the PsdVoigt1 function have the following meaning: y_{0} is the Y-values offset, A is the area, w is the width (FWHM), x_{c} is the center and m_{u} is a profile shape factor.

This command is used to fit a curve with a Pseudo-Voigt function which is a linear combination of Gaussian and Lorentzian functions with different FWHM:

The parameters of the PsdVoigt2 function have the following meaning: y_{0} is the Y-values offset, A is the area, w_{G} is the Gaussian FWHM,
w_{L} is the Lorentzian FWHM, x_{c} is the center and m_{u} is a profile shape factor.