Follow up on "Add various horizontal lines to a plot" Asked by FC93 on 7 Oct 2016

Can I add a horizontal line of specified length below a plot? I want to add a time calibration bar below a heatmap figure. It is easy to superimpose the calib line onto the figure, as per the answers provided to FC93 in his/her query of 7 Oct 2016. But how about below a figure or plot?