Just enter the start and end points of the line and use plot. If you want the line to be at x = 5, and if you want the line to go all the way from the top of the chart to the bottom, do this:
candle(col2,col3,col4,col1)
x = [5 5];
y = ylim; % (gets current y limits)
hold on
plot(x,y,'-','color','red','linewidth',2)
