Drawing the top axis (box) line
问题 I have a plot with two lines and two different x-axis (different data units), which I plot like the following. My problem is that I would like to draw the top line of the box black as well (horizontally), and not leave it "open" like it is. It would be great if the line had the x-axis ticks as well, same as the bottom horizontal axis line. Obviously, grid on doesn't work, because it draws the y1-axis ticks on the right and the y2-axis ticks on the left, which I don't want. Also, I think in