Sometimes when using low-level graphics functions it is needed to convert from bar number to pixel X co-ordinate and from price level to pixel Y co-ordinate. Converting between them needs knowing visible bar range, Y-axis value range and pixel dimensions of drawing area. Once these params are known it is just a matter of performing simple scale transformation. The code example below shows how to do that.
function GetVisibleBarCount()
{
lvb = Status("lastvisiblebar");
fvb = Status("firstvisiblebar");
return Min( Lvb - fvb, BarCount - fvb );
}
function GfxConvertBarToPixelX( bar )
{
lvb = Status("lastvisiblebar");
fvb = Status("firstvisiblebar");
pxchartleft = Status("pxchartleft");
pxchartwidth = Status("pxchartwidth");
return pxchartleft + bar * pxchartwidth / ( Lvb - fvb + 1 );
}
function GfxConvertValueToPixelY( Value )
{
local Miny, Maxy, pxchartbottom, pxchartheight;
Miny = Status("axisminy");
Maxy = Status("axismaxy");
pxchartbottom = Status("pxchartbottom");
pxchartheight = Status("pxchartheight");
return pxchartbottom - floor( 0.5 + ( Value - Miny ) * pxchartheight/ ( Maxy - Miny ) );
}
Plot(C, "Price", colorBlack, styleHistogram );
GfxSetOverlayMode(0);
GfxSelectSolidBrush( colorRed );
GfxSelectPen( colorRed );
AllVisibleBars = GetVisibleBarCount();
fvb = Status("firstvisiblebar");
for( i = 0; i < AllVisibleBars ; i++ )
{
x = GfxConvertBarToPixelX( i );
y = GfxConvertValueToPixelY( C[ i + fvb ] );
GfxRectangle( x-1, y-1, x + 2, y+1 );
}
RequestTimedRefresh(1); // ensure 1 sec refresh
Note that when chart scale changes, it will usually require one extra refresh to get low-level graphics alignment to new scale. That’s why we added RequestTimedRefresh call at the end.