Curve fitting: OverflowError when a fitted value is far outside the parameter slider range #76

Describe the bug

In the curve fitting dialog (plotpy.widgets.fit), running the automatic fit may raise an unhandled exception when a fitted parameter value ends up far outside the [min, max] range of its slider:

Traceback (most recent call last):
  File "plotpy/widgets/fit.py", line 749, in autofit
    prm.update()
  File "plotpy/widgets/fit.py", line 354, in update
    self.update_slider_value()
  File "plotpy/widgets/fit.py", line 328, in update_slider_value
    self.slider.setValue(intval)
OverflowError: argument 1 overflowed: value must be in the range -2147483648 to 2147483647

Besides the error message, the dialog is left in an inconsistent state:

• the slider of the faulty parameter stays unresponsive: the exception is raised between blockSignals(True) and blockSignals(False), so moving the slider no longer changes the value;
• the fields of the parameters following the faulty one are not updated, while the fit curve already shows the new values.

To Reproduce

1. Open a fit dialog on a Gaussian peak (e.g. y = 5 exp(-x²/2) on [-10, 10], with parameters ranges of a few units)
2. Click "Bounds" and restrict the fit range to the tail of the peak, e.g. [8, 10]
3. In "Settings", choose the Powell method
4. Click "Run"
5. The amplitude diverges and the exception above is raised (the computed slider position is about 1e14)

Only L-BFGS-B uses the [min, max] bounds of the parameters: the other methods (including the default least squares method) are unconstrained, so any ill-posed fit may move a parameter far away from its slider range.

Expected behavior

The fit result is displayed without error: the slider stays at the nearest end of its range, remains usable, and all parameter fields show the fitted values. A value outside the slider range should be visible as such.

Additional context

The cause is in FitParam.update_slider_value, which converts the value into a slider position without clipping it to the slider range:

intval = int(self.steps * value_delta / total_delta)
self.slider.setValue(intval)

With steps = 5000, a value about 430 000 times the range width away from min exceeds the 32-bit range of QSlider.setValue. The same code path fails in other situations:

• min == max and a different value: division by zero, then OverflowError: cannot convert float infinity to integer (or ZeroDivisionError when the dialog is created)
• log scale with a value below min - 1: ValueError: cannot convert float NaN to integer
• nan or inf typed in a parameter field: accepted as values, then ValueError/OverflowError
• a "Steps" value of 1 in the parameter properties dialog: ZeroDivisionError when the slider is moved