// Downloaded From https://www.WiseStockTrader.com
// in reply to: http://forum.amibroker.com/t/afl-to-draw-trendline-between-previous-day-high-and-previous-day-low/2445
// code assumes your working with intraday data (e.g. 5min data), 10/2017, E.M.Pottasch

lineWidth = Param( "Linewidth", 1, 1, 10, 1 );
zorder = Param( "Zorder", 1, -2, 2, 1 );

dn = DateNum();
newDay = dn != Ref( dn, -1 );
bi = BarIndex();
fvb = FirstVisibleValue( bi );
lvb = LastVisibleValue( bi );

highOfDay = TimeFrameGetPrice( "H", inDaily );
lowOfDay = TimeFrameGetPrice( "L", inDaily );

lbx1 = ValueWhen( newDay, bi );
lbx0 = ValueWhen( newDay, bi, 0 );
ph1 = ValueWhen( highOfDay == H, H );
px1 = ValueWhen( highOfDay == H, bi );
pdd1 = ValueWhen( highOfDay == H, dn );
tl1 = ValueWhen( lowOfDay == L, L );
tx1 = ValueWhen( lowOfDay == L, bi );
tdd1 = ValueWhen( lowOfDay == L, dn );

SetChartBkColor( ColorRGB( 0, 0, 0 ) );
Plot( Close, "Price", colorDefault, styleCandle, Null, Null, 0, 0, 1 );
Plot( newday, "", colorDarkBlue, styleHistogram | styleOwnScale | styleNoLabel | styleNoRescale, 0, 1, 0, 0, 5 );
Plot( highOfDay, "", colorGreen, styleLine, Null, Null, 0, 1, 1 );
Plot( lowOfDay, "", colorRed, styleLine, Null, Null, 0, 1, 1 );

GfxSetZOrder( zorder );
GfxSetCoordsMode( 1 );

cnt = 0; // count number of visible days

for( i = lvb; i > fvb; i-- )
{
    idx = lbx1[ i ];

    if( IsEmpty( idx ) ) break;

    if( pdd1[ idx ] > tdd1[ idx ] )
    {
        hv = ph1[ idx - 1 ];
        hx = px1[ idx - 1 ];
        lv = tl1[ idx ];
        lx = tx1[ idx ];
    }
    else
        if( pdd1[ idx ] < tdd1[ idx ] )
        {
            hv = ph1[ idx ];
            hx = px1[ idx ];
            lv = tl1[ idx - 1 ];
            lx = tx1[ idx - 1 ];
        }
        else
            if( pdd1[ idx ] == tdd1[ idx ] )
            {
                hv = ph1[ idx ];
                hx = px1[ idx ];
                lv = tl1[ idx ];
                lx = tx1[ idx ];
            }

    slope = ( hv - lv ) / ( hx - lx );

    if( hx < lx )
        line = LineArray( hx, hv, lx, lv );
    else
        line = LineArray( lx, lv, hx, hv );

    Plot( IIf( line, line, Null ), "", colorYellow, styleLine | styleNoLabel | styleNoRescale, Null, Null, 0, zorder, lineWidth );

    x1 = Max( hx, lx );

    if( IsEmpty( x1 ) ) break;
    else
    {
        x2 = idx;
        dx = x2 - x1;
        y1 = line[ x1 ];
        y2 = y1 + dx * slope;

        // extension inside same day (dashed yellow)
        GfxSelectPen( colorYellow, lineWidth, 1 );
        GfxMoveTo( x1, y1 );
        GfxLineTo( x2, y2 );

        // extension into next day (dashed violet)
        x1 = idx;

        if( IsEmpty( lbx0[ idx ] ) OR lbx0[ idx ] <= idx )
        {
            x2 = BarCount - 1;
        }
        else
        {
            x2 = lbx0[ idx ];
        }

        dx = x2 - x1;

        y1 = y2;

        y2 = y1 + dx * slope;
        GfxSelectPen( colorViolet, lineWidth, 1 );
        GfxMoveTo( x1, y1 );
        GfxLineTo( x2, y2 );
    }

    i = lbx1[ idx ];
    cnt++;
}

"cnt: " + cnt;