When in parametric mode (`set parametric`), the xrange is as fully scalable as the y range. In other words, in parametric mode the x axis can be automatically scaled to fit the range of the parametric function that is being plotted. Of course, the y axis can also be automatically scaled just as in the non-parametric case. If autoscaling on the x axis is not set, the current x range is used.
Data files are plotted the same in parametric and non-parametric mode. However, there is a difference in mixed function and data plots: in non-parametric mode with autoscaled x, the x range of the datafile controls the x range of the functions; in parametric mode it has no influence.
For completeness a last command `set autoscale t` is accepted. However, the effect of this "scaling" is very minor. When `gnuplot` determines that the t range would be empty, it makes a small adjustment if autoscaling is true. Otherwise, `gnuplot` gives an error. Such behavior may, in fact, not be very useful and the command `set autoscale t` is certainly questionable.
`splot` extends the above ideas as you would expect. If autoscaling is set, then x, y, and z ranges are computed and each axis scaled to fit the resulting data.