Некорректная отрисовка графика на канве

//---------------------------------------------------------------------------
 
#include <vcl.h>
#include<math.h>
#pragma hdrstop
 
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
: TForm(Owner)
{
}
 
float f(float x)
{
return sin(x);
}
void TForm1::Grafik()
{
float xmin,xmax,ymin,ymax;
 
float z,t,a,j;
float x1, x2; 
float y1, y2; 
float x;
float y; 
float dx; 
int l, b; 
int w, h; 
float mx, my; 
int x0, y0; 
 
l = 10; 
b = Form1->ClientHeight-20; 
h = Form1->ClientHeight-100; 
w = Form1->Width -400; 
 
xmin=StrToFloat(Edit3->Text);
xmax=StrToFloat(Edit4->Text);
dx=StrToFloat(Edit5->Text);
 
 
x=xmin;
ymax=f(x);
ymin=f(x);
 
z=700;
t=75;
a=850;
j=75;
 
 
do
{
y=f(x);
if (y<ymin) ymin=y;
if (y>ymax) ymax=y;
Canvas->TextOutA(z-7,t, (x));
Canvas->TextOutA(a-50,j, (y));
Canvas->MoveTo(z-10,t-5);
 
 
x+=dx;
 
t+=20;
j+=20;
 
Canvas->LineTo(z-10,t-2) ;
Canvas->LineTo(a+50,j-2);
 
}
while (x<=xmax);
z=700;
t=75;
 
Canvas->MoveTo(900,j-2);
Canvas->LineTo(900,70);
Canvas->LineTo(900,70);
Canvas->LineTo(z-10,t-5);
Canvas->MoveTo(z+90,70);
Canvas->LineTo(z+90,j);
 
Edit1->Text=ymin;
Edit2->Text=ymax;
 
x1 = -3; 
x2 = 3; 
dx = 0.314; 
 
x = x1;
y1 = f(x); 
y2 = f (x); 
 
do
{
y = f (x);
if ( y < y1) y1 = y;
if ( y > y2) y2 = y;
x+= dx;
}
while (x <= x2);
 
my = (float)h/abs(y2-y1); 
mx = w/abs(x2-x1); 
 
x0 = 1+abs (x1*mx) ;
y0 = b-abs (y1*my) ;
Canvas->MoveTo (x0+5,b) ; Canvas ->LineTo (x0+5,b-h) ;
Canvas ->MoveTo (l-2,y0) ;Canvas->LineTo (l+w,y0) ;
Canvas->TextOutA(x0+5,b-h, FloatToStrF(y2, ffGeneral, 6,3));
Canvas->TextOutA(x0+5,b, FloatToStrF(y1, ffGeneral, 6, 3)) ;
 
x = x1;
Canvas->Pixels [x0+x*mx] [y0-y*my] = clNavy;
Canvas->MoveTo ((x0+x*mx),(y0-y*my));
 
do
{
y = f (x);
Canvas ->LineTo ((x0+x*mx),(y0-y*my)) ;
x+= dx;
}
while (x <= x2) ;
Edit1->Text=ymin;
Edit2->Text=ymax;
 
}
 
 
 
//---------------------------------------------------------------------------
void __fastcall TForm1::FormPaint(TObject *Sender)
{
Grafik();
}
 
void __fastcall TForm1::FormResize(TObject *Sender)
{
TRect ret = Rect(0,0, ClientWidth, ClientHeight) ;
Canvas ->FillRect(ret); 
Grafik();
}
 
 
void __fastcall TForm1::Button1Click(TObject *Sender)
{
Grafik() ;
}
//---------------------------------------------------------------------------