Graphing 2 separate data files on one graph

I know its not that difficult graphing 2 things on one plot using gnuplot in linux but this seems like a unique situation that ive been trying to figure out for about a week. I've tried many things that each sort of worked so bare with me as I try and explain:

I have 2 things I want to graph on the same chart; I have graphed them individually, but each time I graph them together I have issues.

1) I tried using multiplot and replot...with multiplot, they become graphed together but they each have their own scales overlapping on the x and y scales.. **so my first question is how can I graph them together with only one x and y scale?**

With replot, the second thing i graph becomes tiny because the scales are not right and I don't know how to fix them to accommodate both..**any suggestions?**

2)I tried writing a script which I then loaded in gnuplot, but it would not plot both..i believe it was because they have different x and y values although they fall on the same scale... **is there anything other than replot and multiplot**

This next part is a second issue that may make the first^ issue easier if I fix it:

The first thing to graph is pretty straightforward:

The first code for the first one is

plot [1:3] [12:19] "results1" 6:12 which I can successfully graph (its just 2 columns of a data file)

The problem is with the second thing I want to graph... maybe there's a better way to graph the second one to make replot or multiplot work, or use a simpler method:

Basically I want to graph something with the equation y=mx+b ... I have m and b as a constant # and x is one column of a data file (which I copied into its own). I tried a bunch of codes to graph this, but it didnt work so I took two values for x, plugged it in and got y and then made a datafile with hose 2 points and graphed that...the problem this causes is that the line is not extending beyond those points, which is why I am having issues with the scale so:

a) does anyone know a better code to graph y=mx+b, x being a bunch of points in a data file and y being unknown, or

b) if I keep it as two data points, does anyone know of a way to extend that line of that slope further beyond the 2 i have used?

i feel like the solution could be easy, its just I've tried multiple things each with its own problems so its now complicated.. thanks