Wave with Y values at a single X value from a graph
Function extractGraphAtXValue(theX, theGraph)
    Variable theX
    String theGraph

    String outwname = theGraph+"at"+num2str(theX)
    outwname = CleanupName(outwname, 1)
    extractGraphAtX(theX, theGraph, outwname)
end

Function extractGraphAtX(theX, theGraph, outwname)
    Variable theX
    String theGraph
    String outwname
   
    if (strlen(theGraph) == 0)
        theGraph = WinName(0,1)
    endif
   
    String tlist = TraceNameList(theGraph, ";", 1)
    Variable i
    Variable ntraces = ItemsInList(tlist)
    Variable pnt
   
    Make/D/O/N=(ntraces) $outwname
    Wave outw = $outwname
   
    for (i = 0; i < ntraces; i += 1)
        String oneTrace = StringFromList(i, tlist)
        Wave w = TraceNameToWaveRef(theGraph, oneTrace)
        Wave/Z xw = XWaveRefFromTrace(theGraph, oneTrace)
        if (WaveExists(xw))
            // assume monotonic X values
            pnt = BinarySearch(xw, theX)
        else
            pnt = x2pnt(w,  theX)
        endif
        outw[i] = w[pnt]
    endfor
end


This function extractGraphAtXValue() extracts one Y value at a given X value from every trace on the named graph. If theGraph is "" then it works on the top graph window. It creates a wave with the name derived from the graph name and the X value.

The function extractGraphAtX() allows you to specify your own output wave name.

These functions will work on a mix of both XY and waveform traces.

Note that it does not check that all the traces have sufficient points- any short trace will contribute the value at the last point.

A nice enhancement would be to allow a filter string to select only traces with certain naming convention.

To get the Y values from a graph at the X value of the round (A) cursor in the top graph, you would use the function like this:

extractGraphAtXValue(hcsr(A), "")

This code will add a dynamic update- call SetupDynamicYatXforGraph() to add the A cursor to the graph, make a new graph with the output wave, and set up a window hook on the graph to update the output graph any time the cursor is moved.
Function SetupDynamicYatXforGraph(theGraph)
    String theGraph
   
    if (strlen(theGraph) == 0)
        theGraph = WinName(0,1)
    endif
   
    Cursor/H=2/L=0/P A $(StringFromList(0, TraceNameList(theGraph, ";", 1))) 0
   
    SetWindow $theGraph hook(YatXCursorHook) = YatXCursorHook
    String outwname = "DynamicYatX"+theGraph
    SetWindow $theGraph userdata(YatXDynamicName)=outwname
    Make/N=0/D $outwname
    Display $outwname
    extractGraphAtX(hcsr(a), theGraph, outwname)
end

Function YatXCursorHook(s)
    STRUCT WMWinHookStruct &s
   
    String outwname = GetUserData(s.winName, "", "YatXDynamicName")
    extractGraphAtX(hcsr(a), s.winName, outwname)
end

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