[timeless]

Целесообразно использовать массив.

Код:

procedure TForm1.Button1Click(Sender: TObject);
var
  i: integer;
  ar: array [0..5] of real;
  min, max, scale: real;
begin
  ar[0]:=strtofloat(edit1.Text);
  ar[1]:=strtofloat(edit2.Text);
  ar[2]:=strtofloat(edit3.Text);
  ar[3]:=strtofloat(edit4.Text);
  ar[4]:=strtofloat(edit5.Text);
  ar[5]:=strtofloat(edit6.Text);
  min:=0;
  max:=0;
  for i:=0 to length(ar)-1 do begin
    if ar[i]>max then max:=ar[i];
    if ar[i]<min then min:=ar[i];
  end;
  scale:=image1.Height/(max-min);
  image1.Canvas.MoveTo(0, image1.Height-round((ar[0]-min)*scale));
  for i:=0 to length(ar)-1 do begin
    image1.Canvas.LineTo(round(i*image1.Width/(length(ar)-1)), image1.Height-round((ar[i]-min)*scale));
  end;
end;