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. Results are be given in the Log panel

If the *Apparent Fit* option is checked, QtiPlot uses the apparent values for fitting, according to the current axis scales.
For example, select this box to fit exponentially decaying data with a straight line fit when data are plotted on a log scale.
When this check box is selected and the data has error values associated with it, QtiPlot uses the larger of the positive/negative errors as weight.
Apparent Fit is only useful when you fit from a graph and change the plot axis type (from Linear to Log10, for example).
If you check this option, QtiPlot will first transform raw data into new data space as specified in the graph axis type,
and then fit the curve with the new data. Otherwise, QtiPlot always fits raw data directly, regardless of the axis type.
Apparent fit is equivalent to direct fit if you first transform raw data on the worksheet, and leads to completely different results from direct fit if your graph axis is non-linear.

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.