Depending on what version of Mathcad you can also put up to two vertical or horizontal lines in a plot. Double click on the plot, in the XY axis tab, if you have a selection for "show markers", check the box under x-axis. This will put two placeholders under the x axis. If you type a number in there (won't accept a variable unfortunately), it will draw a vertical line at that point. I think this was added in V13.
Peter