Chi2 map - calculate chi2 as a parameter varies

Function chi2generator(fn,ydata,xdata,coefs,whichParam,searchArea, [edata, lhs, rhs])
    FUNCREF chi2fitfuntemplate  fn
    Wave ydata,xdata,coefs      //y data, x data, and coefficient wave
    variable whichParam, searchArea  //which of the parameters to you want to vary, how far from the original value do you want to search (%)
    Wave edata   //wave containing standard deviation for the x,y pair
    variable lhs, rhs    //specify a region of interest in the data using a left hand side and right hand side.

    variable originalvalue = coefs[whichparam]
    variable range = originalValue * searchArea/100
    variable ii

    duplicate/o ydata, :theoretical_data, :chi2_data
    Wave theoretical_data, chi2_data

    make/o/n=200/d chi2_map
    setscale/I x,  originalvalue - range/2, originalvalue + range/2, chi2_map
   
    if(paramisdefault(lhs))
        lhs = 0
    endif
    if(paramisdefault(rhs))
        rhs = numpnts(ydata)
    endif

    for(ii=0 ; ii < numpnts(chi2_map) ; ii+=1)
        coefs[whichparam] = pnt2x(chi2_map, ii)

        theoretical_data = fn(coefs, xdata)
        chi2_data = (ydata-theoretical_data)^2
       
        if(!paramisdefault(edata))
            chi2_data /= edata^2
        endif
        Wavestats/q/R=[lhs, rhs] chi2_data
       
        chi2_map[ii] = V_avg * V_npnts
    endfor

    coefs[whichparam] = originalvalue

    display/K=1 chi2_map

    killwaves/z theoretical_data, chi2_data
End

Function chi2fitfuntemplate(w, x)
Wave w
variable x
End