Ttdfjt

curveData = Import["D:\Curve.txt", "Table", "FieldSeparators" -> " "]
lineData = Import["D:\Line.txt", "Table", "FieldSeparators" -> " "]
curvePlot = ListPlot[curveData, PlotRange -> 200, 600, All, AxesOrigin -> 200, 0, Frame -> True, True, True, False]
linePlot = ListPlot[lineData, PlotRange -> 200, 600, All, PlotStyle -> PointSize[0], AxesOrigin -> 200, 0, Frame -> False, False, False, True, Filling -> Axis, FillingStyle -> Thickness[Tiny]]
Overlay[curvePlot, linePlot]

I am trying to overlay two plots. However, overlay does not produce aligned plots. The reason seems to be that the plots don't have the same absolute X range, despite the same PlotRange. Could you help to comment the reason ? Many thanks !

Curve.txt

Lines.txt

curveData, lineData = Import["https://pastebin.com/raw/SEg5tThi", "RawJSON"];

Here's a way to combine the plots and keep the frame scales that seem intended by the OP. It would have been easier if Charting`ScaledTicks[curveScale, curveScaleInv] didn't insist that log-spaced major ticks were the way to go. One can use Charting`ScaledTicks in place of myScaledTicks in many cases.

curveData, lineData = Import["https://pastebin.com/raw/SEg5tThi", "RawJSON"];

(* creates scaling functions
 * to scale data to canonical 0,1 interval and back *)
ClearAll[scalingFunctions];
scalingFunctions[data_] :=
 Function /@ (Rescale[#, #2, #3], Rescale[#, #3, #2] &[#, MinMax@data, 0, 1]);
curveScale, curveScaleInv = scalingFunctions[curveData[[All, 2]]];
lineScale, lineScaleInv = scalingFunctions[lineData[[All, 2]]];

(* creates ticks corresponding to scaling functions *)
ClearAll[myScaledTicks];
myScaledTicks[sf_, isf_][min_, max_, n_: 6, 6] := 
 Module[major, minor,
 major, minor = FindDivisions[isf@min, max, n] /. x_Rational :> N[x];
 minor = Complement[Join @@ minor, major];
 major = 
 sf[#], #,
 0.01, 0.,
 AbsoluteThickness[0.1]
 & /@ major;
 minor = 
 sf[#], Spacer[0, 0],
 0.005, 0.,
 AbsoluteThickness[0.1]
 & /@ minor;
 Join[major, minor]
 ];

Plot of OP's data:

Show[
 curvePlot = ListPlot[curveData, PlotRange -> 200, 600, All, 
 AxesOrigin -> 200, 0, Frame -> True, 
 ScalingFunctions -> curveScale, curveScaleInv],
 linePlot = ListPlot[lineData,
 PlotRange -> 200, 600, All, PlotStyle -> PointSize[0], 
 AxesOrigin -> 200, 0, Filling -> Axis, 
 FillingStyle -> Thickness[Tiny], 
 ScalingFunctions -> lineScale, lineScaleInv],
 FrameTicks -> 
 myScaledTicks[curveScale, curveScaleInv],
 myScaledTicks[lineScale, lineScaleInv],
 Automatic, Automatic,
 FrameLabel -> "curve (units?)", "line (units?)", "x (units?)", None
 ]

curveData, lineData = Import["https://pastebin.com/raw/SEg5tThi", "RawJSON"];


curvePlot = ListPlot[
 curveData
 , PlotRange -> 200, 600, All
 , AxesOrigin -> 200, 0
 , Frame -> True, True, True, False
 ];
linePlot = ListPlot[
 lineData.DiagonalMatrix[1, 40000]
 , PlotRange -> 200, 600, All
 , PlotStyle -> PointSize[0]
 , AxesOrigin -> 200, 0
 , Frame -> False, False, False, True
 , Filling -> Axis
 , FillingStyle -> Thickness[Tiny]
 ];

Show[curvePlot, linePlot]

Try

Show[curvePlot, linePlot]

As you can see the scaling of your data should be adapted!

If you want a more automated version of @MiachelE2's answer, you can use the CombinePlots function of the ForScience` paclet (see below for instructions on how to install it) with the setting "AxesSides" -> "TwoY" (see the documentation of the paclet for more details and examples):

<<ForScience`

curveData, lineData = Import["https://pastebin.com/raw/SEg5tThi", "RawJSON"];

curvePlot = ListPlot[
 curveData,
 PlotRange -> 200, 600, All,
 AxesOrigin -> 200, 0,
 Frame -> True,
 FrameLabel -> "x axis", "curve (units?)"
 ]
linePlot = ListPlot[
 lineData,
 PlotRange -> 200, 600, All,
 PlotStyle -> PointSize[0], Red,
 AxesOrigin -> 200, 0,
 Filling -> Axis,
 FillingStyle -> Thickness[Tiny],
 Frame -> True,
 FrameLabel -> "x axis", "line (units?)",
 FrameStyle -> Red
 ]

CombinePlots[curvePlot, linePlot, "AxesSides" -> "TwoY"]

Note how the FrameLabel,FrameTicks and FrameStyle of the second plot were automatically moved to the right side

How to install the paclet

The current version (0.88.45) of the paclet can be installed by running

PacletInstall[
 "https://github.com/MMA-ForScience/ForScience/releases/download/v0.88.45/ForScience-0.88.45.paclet"
]