The figure below suggests how you might begin using scroll bars, to 'eyeball fit' some data.
 

The upper slider is linked to cell D2, and the lower slider is linked to D4.

Under Insert / Name / Define, cell C2 has been named a,   and cell C4 has been named b .

In cell C2 has been entered '=D2/20'.
This gives a slope from 0 to 5 when the upper slider runs from 0 to 100

In cell C4 has been entered  '= -2 + D4/16'.
This gives an intercept which can run from -2 to above +4, when the lower slider goes from 0 to 100.

In C10 has been typed '=A10*a+b'.
This is the  y = ax + b form of a line.
Then cell C10 was copied down through C15.