Accessing variables from other .cpp files (I know how but I'm missing something)

Question

This is one of my source files ThreeDGraph.cpp:

#include "StdAfx.h"
#include "Global.h"
#include "Win32Project5.h"
#include "ThreeDGraph.h"
#include "ClipinData.h"
#include <windows.h>
#include <string>
#include <ios>
#define MAX_LOADSTRING 100


void CThreeDGraph::Draw3DGraph()
{
    CClipinData ClipinData;

    //CThreeDGraph Graph;

    int SetXMaxValue, SetYMaxValue;
    int SetXMinValue, SetYMinValue;

    string TodaysDate;
    string ClipinID = ClipinData.InstallationID;

    int red = rand() % 256;
    int green = rand() % 256;
    int blue = rand() % 256;

    HPEN CustomColourPen = CreatePen(PS_INSIDEFRAME, 1, RGB(red, green, blue));
    HPEN BlackPen = CreatePen(PS_INSIDEFRAME, 1, RGB(0, 0, 0));

    for (int Day = 0; Day < ClipinData.TotalNumberOfDaysOfData; Day++)
    {
        TodaysDate = ClipinData.ClipinDataSet[Day].DateOfStartOfData.FormatDateOnly();

        int parameterID;
        float parameterVAL;
        int SecondsFromStartOfDay = 0;

        //HDC hdcdraw = hdc;

        int xoffset = 0;
        int yoffset = 0;

        SetXMaxValue = 864.4;
        SetXMinValue = 10;
        SetYMaxValue = 900;
        SetYMinValue = 1;

        MoveToEx(hdc, 10 + xoffset, 900 - yoffset, NULL);

        for (int ParameterCount = 0; ParameterCount < ClipinData.ClipinDataSet[Day].totalNumberEvents; ParameterCount++)
        {
            parameterID = ClipinData.ClipinDataSet[Day].EventsList[ParameterCount].parameterID;
            parameterVAL = ClipinData.ClipinDataSet[Day].EventsList[ParameterCount].parameterVAL;
            SecondsFromStartOfDay = ClipinData.ClipinDataSet[Day].EventsList[ParameterCount].TimeFromStartOfSequence;

            if (parameterID == PRIMARY_TEMP_ID)
            {
                SelectObject(hdc, CustomColourPen);
                LineTo(hdc, (SecondsFromStartOfDay + xoffset) / 90 + xoffset, 900 - parameterVAL * 10 - yoffset);
            }
        }

        SelectObject(hdc, BlackPen);
        MoveToEx(hdc, 10, 1, NULL);
        LineTo(hdc, SetXMinValue, SetXMaxValue);
        LineTo(hdc, SetXMaxValue, SetYMaxValue);

        MoveToEx(hdc, SetXMinValue, SetYMaxValue, NULL);
        LineTo(hdc, 300, 600);

        xoffset = xoffset + 10;
        yoffset = yoffset + 10;

    }

}

This is .cpp file that draws the graph.

This is the header file, Global.h, I'm using to make 'hdc' the global variable:

#ifndef SOURCE1_H_
#define SOURCE1_H_

extern HDC hdc;

#endif

The declaration is as follows in Win32Project5.cpp (bad name I know):

#include "stdafx.h"
#include "Win32Project5.h"
#include "Global.h"
#include "ThreeDGraph.h"
#include "GBasic2DGraph.h"
#include "ClipinData.h"
#include <string>
#include <stdlib.h>
#include <vector>
#include <fstream>
#include <iostream>
#include <windows.h>
#include <winbase.h>
#include <direct.h>
#include "Shobjidl.h"

..........................

Line1:  case WM_PAINT:
            InvalidateRect(hWnd,NULL,true) ;
                hdc = BeginPaint(hWnd, &ps);
                    // TODO: Add any drawing code here...
                     {
                        Graph.Draw3DGraph();
                    }

        EndPaint(hWnd, &ps);
break;

And I get the errors:

1. LNK1120 1 unresolved externals

2. LNK2001 unresolved external symbol "struct HDC__ * hdc" (?hdc@@3PAUHDC__@@A)

Win32Project5 F:\Win32Project5\Win32Project5\ThreeDGraph.obj< 1

I don't think it is being declared anywhere else expect in Win32Project5.cpp if you could lend me hand that would be greatly appreciated, I'll edit if you need anymore information.