api로 그래프입니다.

api를 처음 접하는 학생입니다..

용어같은것이 너무 힘들어서 이렇게 주석좀 부탁드릴려고 합니다..
주석과 자세한 설명좀 부탁드립니다.. ㅠㅠ
간단한 설명도 괜찮습니다..
꼭 도움주세요 ㅠㅠ

#include
#include

LRESULT CALLBACK WndProc(HWND,UINT,WPARAM,LPARAM);
HINSTANCE g_hInst;
HWND g_hWnd;
LPSTR lpszClass="ApiDrawGraph";

int APIENTRY WinMain(HINSTANCE hInstance,HINSTANCE hPrevInstance
,LPSTR lpszCmdParam,int nCmdShow)
{
HWND hWnd;
MSG Message;
WNDCLASS WndClass;

g_hInst=hInstance;

WndClass.cbClsExtra=0;
WndClass.cbWndExtra=0;
WndClass.hbrBackground=(HBRUSH)GetStockObject(WHITE_BRUSH);
WndClass.hCursor=LoadCursor(NULL,IDC_ARROW);
WndClass.hIcon=LoadIcon(NULL,IDI_APPLICATION);
WndClass.hInstance=hInstance;
WndClass.lpfnWndProc=(WNDPROC)WndProc;
WndClass.lpszClassName=lpszClass;
WndClass.lpszMenuName=NULL;
WndClass.style=CS_HREDRAW | CS_VREDRAW;
RegisterClass(&WndClass);

hWnd=CreateWindow(lpszClass,lpszClass,WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,
NULL,(HMENU)NULL,hInstance,NULL);
ShowWindow(hWnd,nCmdShow);

while(GetMessage(&Message,0,0,0))
{
TranslateMessage(&Message);
DispatchMessage(&Message);
}

return Message.wParam;
}

typedef struct
{
POINT *pData;
DWORD dwNum;
}GRAPHDATA;

GRAPHDATA GraphData={NULL, 0};

BOOL ReadDataFile(char *filename, GRAPHDATA *pGData)
{
FILE *in;
int x, y;

in=fopen(filename, "r");

if (in==NULL)
{
MessageBox(g_hWnd,"파일 열기 실패!!","에러",MB_OK|MB_ICONERROR);
return FALSE;
}

if (pGData)
{
if (pGData->pData) free(pGData->pData);
pGData->dwNum=0;
}

while(1)
{
if (fscanf(in,"%d%d",&x,&y)==EOF) break;
pGData->pData=(POINT*)realloc(pGData->pData, sizeof(POINT)*(1+pGData->dwNum));
if (pGData->pData)
{
pGData->pData[pGData->dwNum].x=x;
pGData->pData[pGData->dwNum].y=y;
pGData->dwNum++;
}
}

return TRUE;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam)
{
HDC hdc;
PAINTSTRUCT ps;
HBRUSH PointBrush, OldBrush;
int i;
DWORD dwCount;
POINT ptOldpoint, ptClPoint;
DWORD dwScale=20, dwPointSize=10;

switch(iMessage)
{
case WM_CREATE:
g_hWnd=hWnd;
ReadDataFile("data.txt", &GraphData);
return 0;

case WM_PAINT:
hdc=BeginPaint(hWnd,&ps);
PointBrush=CreateSolidBrush(RGB(255,0,0));

SetMapMode(hdc,MM_LOENGLISH);
SetViewportOrgEx(hdc,200,400,NULL);

MoveToEx(hdc, -2000, 0,NULL);
LineTo(hdc, 2000, 0);
MoveToEx(hdc,0,-2000,NULL);
LineTo(hdc,0,2000);

for(i=-2000; i <= 2000; i+=dwScale)
{
MoveToEx(hdc, i, -5,NULL);
LineTo(hdc, i, 5);
MoveToEx(hdc, -5, i,NULL);
LineTo(hdc, 5, i);
}

if (GraphData.pData)
{
ptOldpoint.x=ptOldpoint.y=0;
for(dwCount=0; dwCount < GraphData.dwNum; dwCount++)
{
MoveToEx(hdc, ptOldpoint.x*dwScale, ptOldpoint.y*dwScale,NULL);
LineTo(hdc, GraphData.pData[dwCount].x*dwScale, GraphData.pData[dwCount].y*dwScale);
ptOldpoint.x=GraphData.pData[dwCount].x;
ptOldpoint.y=GraphData.pData[dwCount].y;
}

OldBrush=(HBRUSH)SelectObject(hdc, PointBrush);

for(dwCount=0; dwCount < GraphData.dwNum; dwCount++)
{
ptClPoint.x=(GraphData.pData[dwCount].x*dwScale)-(dwPointSize/2);
ptClPoint.y=(GraphData.pData[dwCount].y*dwScale)-(dwPointSize/2);
Ellipse(hdc,ptClPoint.x,ptClPoint.y,
ptClPoint.x+dwPointSize,
ptClPoint.y+dwPointSize);
}

SelectObject(hdc, OldBrush);
}

DeleteObject(PointBrush);

EndPaint(hWnd, &ps);
return 0;

case WM_DESTROY:
if (GraphData.pData) free(GraphData.pData);
PostQuitMessage(0);
return 0;
}

return(DefWindowProc(hWnd,iMessage,wParam,lParam));
}