yy20=your value;
YCoord=LineArray(LastValue(BarIndex()-20),LastValue(YY20),LastValue(BarIndex()),LastValue(YY20),1);
Plot(YCoord,"",colorViolet,styleDashed|styleNoLabel|styleNoRescale,Null,Null,29);
play around with the bold in ycoord and you can adjust the length etc